Why is Cowboy Bebop's music so important to the show?

The production order is the whole answer. Composer Yoko Kanno wrote much of the music before the pictures existed. In her account, preserved in Rose Bridges' study of the soundtrack: the songs had to be recorded when no visual image of the work was available yet, "so I didn't write the songs side to side with the film; rather, the film adapted itself to the music." Director Shinichiro Watanabe then used the finished tracks as inspiration for episodes, cutting action to the score until the show's rhythm is literally the music's rhythm. The famous opening, "Tank!", began its life as background music Kanno wrote for a battle scene.

Watanabe's philosophy made the method inevitable. "In bebop, players threw away the score and played freely," he said. "They wanted to express themselves freely and started to improvise a lot. Cowboy Bebop's characters are like those musicians: they are free." So the series is organized as music: episodes are called sessions, each carries its own genre, big band jazz, blues, heavy metal, samba, funk, country, hip hop, and the set list is printed on the episode titles themselves.

The band was real. Kanno assembled The Seatbelts for the series, recruiting players from Japan, New York and Paris, a working band with no lead singer and guest vocalists like Steve Conte and Mai Yamane. The name, per the show's own liner-note joke, is for the seatbelts the musicians needed during their hardcore jam sessions. They made seven albums and a live DVD; IGN voted the soundtrack the greatest in anime in 2006. Most anime music illustrates a show. This music is the reason the show moves the way it does.