You may very well be living your life to the fullest, but some people are not, simply because they believe in an afterlife. Take some of my more staunchly religious family, for example. They are by-the-book Christians who will turn off a movie they’re enjoying if it says the word “goddamn” (or “GD”, as they call it). They won’t let their kids read Harry Potter or watch Pokemon because “it’s idolatry and witchcraft” and the Bible says that’s wrong. I strongly suspect that one of my relatives is gay but has so thoroughly repressed it with religion that he’d never admit the possibility, probably not even to himself.
Some people genuinely believe that if they can only suffer through this life abiding by all God’s rules, they’ll be in paradise for all eternity. They want to do things and won’t do them because they won’t endanger their chances of spending their afterlife in heaven. Atheism frees us from this unfortunate mindset, and allows us to realize that the only rules that apply to us are the ones that we choose to be subject to, that “good” and “evil” are completely subjective and situational, that your life is YOURS with which to do YOUR will. No, you don’t have to be atheist to see it this way, but atheism basically encompasses and represents these ideas, which is I think what Sleel was trying to say.