I actually found this blog while googling the exact definition of the term "just saying," but here's my two cents. I generally use it as a sort of disclaimer, a verbal 'throwing up the hands in innocence' if you will. I use it after a joke or tease directed at a friend. On one hand it's a disclaimer, but it's also a way to let them know it *was* a joke, and not an actual mean-spirited comment.