"uniqueness" for lack of a better word to describe the way your character differs from my character. Your dude vs my guy. The importance of character "building" in a way that actually matters, but not to be confused with autistic "buildfag" games that focus on very minor differences that have little impact on how the actual game plays out.
For example, how different is your Geralt from my Geralt in Witcher? Is this important? Would the ability to make a more varied Geralt make it a better game? Perhaps the ability to make your Geralt distinct goes beyond stats on the character sheet to choices you make in the game β as the Witcher games tend to be rather narrative heavy and the way our Geralts are different depends not on how we "build" our character but the choices we make when interacting with the world itself?
Therefore following from the last part, the importance of so-called "builds" could possibly be defined in the difference in which they allow us to interact with the fictional world, which is why despite games like Wrathfinder having (supposedly) a very wide range of "builds" they don't seem to make that big of a difference on the game itself.
Perhaps RPGs are missing a link between character building and "narrative building"(again, for lack of a better term)? Can you think of good counterexamples? I could see someone reflexively saying "stat/skill/etc checks", but that becomes more like setting your train car on a specific train track and racing to the end, there is too much "narrative building" lost. For an extreme version of this, see Age of Decadence.

