If God doesn't exist, by definition life cannot have an objective purpose. Morality would have no grounding, and if modern science is right, at some point in the finite future all human accomplishment will be destroyed. Hard to find any meaning, purpose, value, anything in an atheistic world.
If we take atheism to also include physicalism and naturalism, there also basically wouldn't be free will, meaning that we are here in no way by our choice, and that the end results of our lives are predetermined by biology.