I believe that's called chain selvedge (oo, look
) and is usually used to make the edge of an item look nice and even (another link
). I've always heard that the best way to keep your stockinette from curling was to use a 3 st border of garter or seed stitch around the edges you want to flatten. This is the first I think I've heard of slipping the first stitch as a method of keeping stockinette from rolling. It's pretty as well as functional!