Why do the biggest stars bother to play in doubles tournaments (and lose)?

I'm a somewhat new tennis fan, so please forgive me if the answer is obvious. But I see the biggest names compete in doubles matches early in tournaments and often get knocked out in the first round. Why do they bother to do this? Seems like you are exposing yourself to more fatigue and/or injury risk with minimal benefit. Or, if they intentionally take it easy, why bother to compete? Just curious! Thanks for teaching me!