I have my mollies in tanks without salt and they do fine. Others, as you've read, put salt in their tanks because they feel mollies do better in it. Mollies, like many common fish you see in petstores, have been commercially bred in pure freshwater for so many generations that they've adapted well to freshwater only conditions. So really, it's up to you if you want to add salt or not. It's just the way the hobby is, there's a plethora of information out there that are sometimes even contradictory. In time you'll have enough experience to know what works for you and the fish you keep.