Peacock's impressive tail does not make it a much better flyer, or fighter, or whatever. The huge feathers only demonstrate that the bird can still function adequately even with this useless and expensive ornamentation.

The same way, a "highly athletic" / bodybuilder body demonstrates that a man can spend resources on building all these impressive-looking muscles and can strictly ration carbs. (I don't even try to mention what women have to do to look "like a model".)

So it's the same demonstrative behavior, only the results are harder to buy directly than a supercar or a yacht. It's about as practical: not completely useless, but the point is not in the utility but in the hard-to-achieve aesthetics.

Much like a real able flyer looks much less impressive than a peacock (look at a goose or a crow), a real high-achieving athlete looks much less impressive than an athlete for show. But looks sometimes play a critical role, e.g. for an actor, or during dating, so it's not irrational behavior.