Getting into a sling position from prone is sometimes easier to do when entered from the side. It's more difficult to sling up face down than on your side where you have more room to fully engage the sling. The slow roll just helps you get stretched out into a bone support position as opposed to flopping around on the ground like a beached whale.
When they show it in the movies it makes the shooter look cool and causes folks to ask why he did that too.
