Knowing a character’s backstory can be relevant to how they’re played. The backstory on the major faction NPCs in Petals and Thorns was given alongside their identity and motivation because that backstory played an important role in shaping their motivation and identity. Part of the reason the Lunatic Court ran on so long is because I was never quite satisfied with the chain of cause and effect that resulted in a Harlequin who is driven by equal parts fear and mistrust of others on the one hand and a general compassion for other people on the other. Things always seemed to be loaded too far one way or the another.
So what I’m saying here is that when I say that Paizo has a weird habit of drowning us in totally irrelevant backstory, it’s not because backstory is somehow inherently irrelevant. NPCs whose decision-making is important to the story and who interact with the PCs need to have a solid backstory informing their current motivations and why they do the things they do. The problem is that Paizo loves to load its villains up with a whole lot of backstory and then have them appear exclusively for boss fights. If the only purpose of a villain is to provide a climactic encounter at the end, then the only thing that matters is their current scheme. “Nualia is an aasimar corrupted by the Runelords’ power who seeks to use that power to destroy the town of Sandpoint.” Bam, done. If we’re never going to have a conversation with her, we don’t need to care why she hates Sandpoint. Her ultimate goal is to raze it and that’s all that matters.
It all feels very cargo cult-y, like, roleplay-driven games have villains with long backstories, so we’ll set aside a page or two for the backstory of each module’s villain and that’ll make it more roleplay-ish! But, no. Roleplay-driven games include backstories because they’re structured such that the backstories are relevant. You actually need to know what the NPC’s motivations are because the PCs can actually influence their decision-making or at least get to talk to them long enough for the NPC to explain their point of view. Without that, background is just an outline of a short story that will never be written and that only the GM will even know about.