It allows you to add some pressure to the patty while providing it a restricted space in which it can expand. By doing so the ingredients seem to form a much st

It allows you to add some pressure to the patty while providing it a restricted space in which it can expand. By doing so the ingredients seem to form a much stronger bond (from my experience). I used to do the same with beef when I ate it. Also, rolling it into a ball and then wrapping before flattening gives a much better shape to the resulting patty