One reason I was told many years back was that if you leave your seams open the batting can come through the seams during a wash. I wonder if todays batting would do that?
A second reason is ease of butting up the seams. Matching your seams is so much easier.
Of course ironing towards the dark is always a reason as well.
I've heard some say a quilt lays flatter if you iron the seams open, but I was taught to iron them to one side, and I'm afraid of the quilt police, lol.