In the same way that women put on makeup for themselves. However, the desire to be attractive "for ourselves" is baked into our genes precisely because it helped our ancestors attract members of the opposite sex too.

For all the midlife crises, it doesn't really help someone who's not already confident/sexy/rich, though. It's an expensive accessory, like a watch. Makeup is a fair analogy, in that men like to impress other men with their cars and (most) women don't bother being made-up everyday for men.