Three possibilities come to mind:
Is there an evolutionary purpose?
Does it arise as a consequence of our mental activities, a sort of side effect of our thinking?
Is it given a priori (something we have to think in order to think at all)?
EDIT: Thanks for all the responses! Just one thing I saw come up a few times I’d like to address: a lot of people are asking ‘Why assume this?’ The answer is: it’s purely rhetorical! That said, I’m happy with a well thought-out ‘I dispute the premiss’ answer.
No, we don’t know the brain is making use of any quantum phenomena. At best if there is any quantum phenomena in the brain it would just contribute noise. The idea that interference phenomena is actually made use of in the brain for computation is just not backed by anything.