I don't think that there is a single good answer to your question.
The reasons for each performance would probably be different.  I know
that Berlioz placed the brass in a more prominent position that was
contrary to the norm for his Symphonie fantastique, but I don't know of
any period-specific varitions on the orchestra seating that is necessarily
based on the music itself.  More often, it is motivated by the house and
stage conditions.  Other reasons could include positioning sections
together to hear one another better, and providing a comfort level for the
conductor.  I'm sure you could come up with a thousand other plausible
reasons for moving people around like that.

Jonathan Knapp