I don't know the techincal reason for doing it but MY reason for doing it is because I use so much starch my fabric doesn't really fold back willingly so I set the seam with a little steam and a press and then while it is still warm I open the seam up and press it back. The little bit of steam that it gets causes the fabric to soften just enough to fold it back without issue. I never set seams before I started using starch.