Overleaf allows you to work with several authors at the same time on the document. I was skeptical (after all, git allows that too). But after using that feature, well, it is just the killer thing. Now Overleaf doesn't add much on top on my own Latex installation. But the collaborative thing is just spot on (much like Google docs which I have discovered not a long time ago; same analysis : not that good presentation tool, but working together is just incredibly useful).