|This article is about a/an episode in Kamen Rider Drive.|
Why Must They Fight? (彼らはなぜ戦わなければならなかったのか Karerawa naze Tatakawa nara nakatta no ka) is the forty-sixth episode of Kamen Rider Drive. This episode features the debut (and only) appearance of Kamen Rider Chaser Mach, a fusion form between Kamen Riders Mach and Chaser, as well as the proper debut of the Sigma Circular in mecha form. This also marks the final appearance of Medic, Tenjuro Banno, and the Reaper Legion.
Chase protected Go in exchange for his life. Only after seeing him perish does Go realize the impact Chase had on him. Then, Gold Drive, whom Chase supposedly used his final strength to defeat, appears before him. Meanwhile, Shinnosuke heads for the rooftop of the SSDC alongside Heart and Medic. A device that would turn humanity into data, the Sigma Circular, is there. However, right before they could reach the place, the second Global Freeze hits...
Rinna and Genpachiro manages to cut the building's power source, allowing Shinnosuke and the executives to get in. Despite Chase's death, Gold Drive still survives the kamikaze attempt, prompting Mach to fight him again. Having finally acknowledges Chase as his friend, he used the Signal Chaser Bike to become Kamen Rider Chaser Mach and seemingly defeated Gold Drive, destroying the hijacked body of 006. Kiriko reunites with Go and delighted over his victory but her happiness cut short after discovering Chase's death.
After Drive, Heart and Medic defeated the remaining three Reaper Legionnaires, Medic tried to save their Cores but they instead exploded and soon realized that she had lost this ability ever since she woke up, hinting it as Banno's final trap. Shinnosuke receives a call from Kiriko of Gold Drive's death. As the last two Roidmudes mourn of Chase's demise, the second Global Freeze has taken action, but as they tried to advance, Medic was forced to be left behind after she suddenly weakened not even a second after she re-assumed her Over Evolved form. Meanwhile, Banno appears to have survived in his Driver despite Gold Drive's defeat but before his mind can be transferred again, Go kills him with the Shingou-Ax. Once done, Go gives Kiriko Shift Deadheat and orders her to get to Shinnosuke.
Reaching the Sigma Circular, Drive and Heart arrived too late as Sigma finally evolves and attacks the pair with blistering speed and great power. At the same time, victims of Heavy Acceleration are slowly digitizing from Sigma's power. As the machine utilizes the power of all four Over Evolved Core Driviars, Heart and Drive plan a simultaneous attack but failed. A weakened Medic appears and her body disintegrates but in her final moments, Medic's Core assumes the form of the goddess she was from the first Global Freeze, healing Shinnosuke before evaporating into nothingness. In rage, Shinnosuke transforms into Type Tridoron and stood up to continue the battle to end Banno's madness.
- Shinnosuke Tomari (泊 進ノ介 Tomari Shinnosuke): Ryoma Takeuchi (竹内 涼真 Takeuchi Ryōma)
- Kiriko Shijima (詩島 霧子 Shijima Kiriko): Rio Uchida (内田 理央 Uchida Rio)
- Chase (チェイス Cheisu): Taiko Katono (上遠野 太洸 Kotono Taiko)
- Go Shijima (詩島 剛 Shijima Gō): Yuu Inaba (稲葉 友 Inaba Yū)
- Jun Honganji (本願寺 純 Honganji Jun): Tsurutaro Kataoka (片岡 鶴太郎 Kataoka Tsurutarō)
- Rinna Sawagami (沢神 りんな Sawagami Rinna): Rei Yoshii (吉井 怜 Yoshii Rei)
- Kyu Saijo (西城 究 Saijō Kyū): Kenta Hamano (浜野 謙太 Hamano Kenta)
- Genpachiro Otta (追田 現八郎 Otta Genpachirō): Taira Imata (井俣 太良 Imata Taira)
- Narration (ナレーション Narēshon), Drive Driver's voice (ドライブドライバーの声 Doraibu Doraibā no koe): Chris Peppler (クリス・ペプラー Kurisu Pepurā)
- Heart (ハート Hāto): Tomoya Warabino (蕨野 友也 Warabino Tomoya)
- Medic (メディック Medikku): Fumika Baba (馬場ふみか Baba Fumika)
- Tenjuro Banno (蛮野 天十郎 Banno Tenjūrō, Voice), Sigma Circular (シグマサーキュラー Shiguma Sākyurā, Co-Voice, uncredited): Masakazu Morita (森田 成一 Morita Masakazu)
Shift Cars, Signal Bikes & Viral Cores
- Car Used:
- Type Change: Shift Tridoron
- Tire Exchange: N/A
- Type Used:
- Type Tridoron
- Tire Blending used in Type Tridoron:
- Kouji Genbar
- Bike Used:
- Form Change: Signal Mach, Signal Chaser
- Signal Exchange: N/A
- Form Used:
- Mach, Chaser Mach
- Core Used:
- Chaser Cobra (Chaser Muchi), Chaser Spider (Chaser Tsume), Chaser Bat (Chaser Yumiya) (all are absorbed into Chaser Mach)
- When changing into Chaser Mach, the close up shot of Chaser Mach shows that the left side of Chaser Mach is covered with Chaser's underarmor is slightly different than the preceding and aftershot before it.
- When Chaser Mach does a 360 spin around Gold Drive, the camera angle of the shot shows the suit actor of Chaser Mach actually missing before cutting to an angle where it looked like it actually connected.
- Before Go kills Banno, the Shingou-Ax announces "Matteroyo" before Go even hit the red Shingou Push Button.
- This can mainly be attributed to the editors assuming Yuu Inaba pressed the Shingou Push Button with his palm instead of his index finger.
- As part of Super Hero Time, this episode aired alongside Shuriken Sentai Ninninger episode 29, Shinobi 29: Definitive Edition Ninja Sugoroku!.
- Viewership: 4.7%
- Closing Screen Shift Car/Signal Bike
- Roidmude Count
- Answer to the episode's question: To put an end to Banno's reign of terror once and for all.
- The way Go kills Banno is similar to how Shinnosuke killed Mr. Belt during Kamen Rider Drive: Surprise Future, with the Shingou-Ax being used as the weapon. Unlike Shinnosuke, Go uses a Full Throttle finisher and made sure to completely connect the Shingou-Ax's blade with the Driver, resulting in complete destruction.
- As of this episode, with the deaths of 081, 101, and 105, all Roidmudes who have Core numbers in the 80s and 100s range are eliminated.
- With 101's death, all 38 of the Bat Type Roidmudes have been destroyed. With Medic's, all 32 of the Cobra Types are gone.
- When Chaser Mach defeats Gold Drive and exposes his Roidmude body, the scene is "a high-speed hero with a scarf defeated a skull head guy", including two Shotaro Ishinomori's creations, Cyborg 009 and Skull Man.
- The 2007 Anime The Skull Man also intimated that Mikogami Hayato/Skull Man/Skull is Joe Shimamura's father, just like Banno and Go.
- Tomoya Warabino's (Heart) 28th birthday and director Takayuki Shibasaki's 37th birthday, falling on August 4th and August 6th 2015 respectively, were celebrated during filming of this episode. 
- The airdate of this episode (9/13/2015) is also occurs on "Kaixa Day", a meme holiday partaken by many Kamen Rider fans to celebrate the "inner jerkiness" of Masato Kusaka/Kamen Rider Kaixa.
- The first TV commercial teaser for the upcoming series Kamen Rider Ghost was aired right after the preview of the next episode.
- This episode continues a trend from the previous series where the instigator of the series' events (Wizard's Sou Fueki and Gaim's Ryoma Sengoku & Rosyuo) isn't killed or defeated by the Main Rider.
- W's Ryubee Sonozaki is a zigzagged example, as while Double did break his Terror Memory, he was not the cause of Ryubee's death.
- The location of Sigma's hideout raided by Drive and Heart is also the same location in Gaim where the shrine tree was placed by Yggdrasill Corporation at their headquarters until the tree was restored following the end of Inves' invasion and Yggdrasill's bankrupt.
- Kyu's code number which he receives when the Sigma Circular starts digitizing humans references his doppelganger Roidmude's Core number through the first three digits, 0726152886.
- This is the only appearance of Mach's Chaser form.
- Final appearance of Drive Type Tridoron Kouji Genbar.
- This also marks the last time Drive initiates a standard Tire Blending.
- It also marks the return of Drive Type Tridoron Kouji Genbar since episode 39.
Kamen Rider Drive Volume 12 features episode 45-48: What is a Roidmude's Last Dream?, Why Must They Fight?, Who Will You Entrust the Future to, My Friend? and Final Story (Special Edition): The Case of Ghost. 
Collection 4 comes with 12 episodes.
to be added.