"why is it so important to have a belief that serves no purpose... "
Ok. Atheism is NOT a belief. It is the lack of belief that many people share. If there were no god-believers, there would be no atheists. WE would all be known as people.
Now, why is it so important? Because it allows us to see more truth in our lives and to rely on our own reasoning, courage, strength, and sensitivities, thereby making us stronger, more pure, more compassionate, and more fun to be around. Losing religion, if religion doesn't speak to you, is like losing a harmful addiction. Not a bad thing at all.