It usually comes from crossovers where the authors are pitting the powers of different mythologies against each other. Greek demigod versus Olympian demigoddess. Percy has the whole son of Poseidon, Curse of Achilles thing going on, which in some fan interpretations makes him ridiculously overpowered. They'll have him face off against Ares or something, win, and then Wonder Woman, respecting the rules of a duel or ancient custom, shows deference. It's a power fantasy, but also about merging two worlds where honor and combat rituals matter.
I've seen it used as a narrative shortcut for establishing Percy's 'worthiness' in the wider DC universe. Instead of a long fight, Diana bows, acknowledging a fellow warrior. It can feel a bit forced if not written well, but when it works, it serves as a cool moment of mutual respect between two iconic figures from very different pantheons. Some writers just really want to see their favorite underdog get that kind of recognition.