That's an answer that seems to work within the materialistic worldview. But interactive, supernaturalistic theism has a different answer. For us, bi-directional communication with the higher power is the source of purpose. Inherent purpose cannot arise from materialism -- only self-constructed purpose. And especially for the mentally ill, that which is merely self-constructed is not trustworthy. But if you sense that what you are is guided from on high, the unknown future becomes bearable and hope, which was formerly non-existent, becomes possible. And in the presence of hope, mental illness weakens and therapy is a thing that one can work through to a reachable goal.
This seems to portray a scenario in which the materialists are the strong selfmade mental nobility, who need no crutch, looking down with mixed contempt and pity on the unwashed theistic masses. To which I can only say that pity is also due the blind.