My understanding is that pressing to one side carried over from the days when piecing was done by hand. With hand piecing, you have only a single thread running up and down through two layers of fabric, rather than a top and bottom thread for each stitch. This means there is an actual gap for batting to come through if a seam were to be pressed open. Pressing to one side covers the gaps in a hand running stitch.
That said, I love the sturdy feel and neater look (on the back) that pressing to one side gives a quilt top, and I find I get better (and easier) seam matching when the seams nest.