Most importantly, the humans give the Culture legitimacy as a galactic civilization, because many of the other Involveds don't give a crap about AIs, but they respect the flesh and blood humans. The humans are also a source of wonder and whimsy and surprise for the Minds, with some outlier humans even able to surpass them in some aspects.
The very first Minds were also programmed to like humans, and that aspect has simply stuck, despite their self-evolution into something far surpassing human ability to create.