Not technically related to Bitcoin but I couldn't find a better place to post.

Could someone explain the rationale for requiring a client seed as part of provably fair schemes?

Assuming the server seed commitment is known before placing a bet, the server can't alter the outcome in any way. Is is the idea that a server could predict betting patterns for an individual user (and shuffle cards / pick numbers accordingly)? That's the only thing I could think of which a client seed does prevent.