I have known theist that are uncertain about a great deal of things including the purpose of life and even belief in God himself. Even Soren Kierkegaard writes about uncertainty in religious faith instead of certainly; that is faith with uncertainty. I have also seen atheists who are dead certain about the purpose of life and about the non-existence of gods. So I think you need to just talk plainly and be more specific.
Have you ever heard theist (not agnostic or far in between) be uncertain about god's existence and his/her/their role in that theist's life?
The difference between atheism and theist is theist belief in god. So god gives a theist a
specific certainty and purpose(s) about life to which he believes in god to know what that theist does not know about life.
The atheist doesn't have the
specific certainty and purpose in life based on god because he doesn't believe in one. Therefore, regardless of how strong and what his purpose is, the
benefit to theism is that he will always be uncertain when for many theist god offer's certainty.
Many theist (since Pagans and Hindu and theists too) but not all believe god answers the rest of what they are not certain about.
Compared to an theist, atheist does not have that advantage. Whether that's important or not isn't your question. You just asked what the benefit of theism is.
God offers security in life that a theist would not know or have without him.
God offers a sense of purpose because "god" fills in the blank of what the theist doesn't know about life.
I am an atheist. Whether or not
we have this, how, or why isn't the point. You asked the benefits of theism not compare the benefits of theism to atheism and debate which is correct.
(you asked what benefits theism not what's right or wrong)