Purpose appears to motivate and concern us. There are levels of purpose, from the immediate purpose of a functional task, to the purpose of life itself, whether our individual life, or the totality of all human life. A bird has purpose in building its nest to nurture its young. According to neo-Darwinism, the purpose of our genes is survival and reproduction. But this implies that the only purpose of existence is further existence. Theists insist on a divine purpose for humanity to please God, although this is an egocentric view of deity. But God cannot be both perfect and purposive. Purpose needs to exist independently of God to have any validity and not be simply an arbitrary command.
For the atheist, the only available targets of purpose are humanity and nature. A proposal for an atheist purpose therefore has to aim to deliver benefit to either humanity, nature, or both. Since nature itself defines no purpose, then any purpose is humanly defined and determined. Purpose is voluntary, arbitrary, and self-defined. People may opt for different degrees of purpose. Some people appear content to live relatively purpose free, others are purpose driven. We decide our own purpose. We may choose a purpose to deliver creative output, pleasure, enjoyment, good, happiness, well-being, both to ourselves and to others, as these are the ultimate human outcomes. We can also choose a purpose of preserving and cultivating nature. We have the option to reject purpose and simply live, but purpose in life is enriching.