Brook's backstory has a unique structure compared to other flashback sections. Firstly, it happens at the end of the arc instead of someplace in the middle. Most flashbacks are meant to clear up mystery and inject one final shot of emotional energy to carry the end of the arc through the big fight scenes. Here, we already know everything that happens to Brook; nothing is a surprise. Instead we're learning how it happened.
Secondly, Brook's backstory is the only one that I can think of that intermittently bounces back to the present. In fact, it actually goes on three timelines: Brook alive with the Rumbar Pirates, Brook alone and dead during his fifty years of isolation, and the present day party with the Straw Hats.
The thread that connects these three sections is Bink's Sake. Music is the bridge between past and present, and the instrument by which Brook is allowed to grieve and put his former crew to rest.
Back during the Skypiea arc music was portrayed as a connecting force. I noted at that time all the chapter names that were musically themed (there were a lot) and the ringing of the golden bell was literally called a love song that brought peace to a warring land. Bink's Sake is much the same. There's a reason why Luffy insisted for so long that he needed a musician on the crew.
(As an aside, in an upcoming SBS Oda mentions how he wrote and commissioned the music for Bink’s Sake 4-5 years before it came up in the manga, anticipating that he would need it soon for the story. While Oda is notoriously bad at planning out timelines, it does mean that he was thinking about Brook and Bink’s Sake at the end of Skypiea. Perhaps at one time he was planning on going from the musical theming of the Skypiea chapter titles to actual, in-universe music back to back. We’ll probably never know for sure.)
Ages ago I wrote a long meta on Brook, and at that time I noted that there is nothing strictly necessary about Brook. The Straw Hats have another, better swordsman, and his skill isn't needed for sailing along the open seas.
But the Straw Hat Pirates without Brook is like life without music. You can do it, but it's a life without joy and bereft of the simple connections born from singing a song with the people you love. Luffy has understood that from the beginning, and it's why Brook is so, so important to the crew.
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