Hm, without spoiling anything, I'd say it's context? Like, on reread, Ace dying def hits harder because you saw their past and their bond and got to know them better, for one. Then, it's supposed to be a breather after Marineford, obviously, and designed to raise questions (why have we never heard about Sabo until now, how will Ace's death impact Luffy when he never seems to change, what happened to the bandits, that sort of thing...). And, since Ace's death marks a turning point in the series, now is the best time to get more insight into our protagonist ofc. But mainly, I think it's just supposed to flesh out their relationship some more so we can empathize better with what Luffy is going through in the present.