Do you need a specific game mechanic for that?

Not really, I suppose. But having it explicit can help players realize they have the option.

"Concede" is also handy for situations where a player feels that their character winning this particular battle would be out of character.

It's a particularly helpful rule for cases where the player wants their character to do something particularly foolish, maybe to reach a specific story outcome, but still wants some influence on the final outcome.

It can go along the lines of:

Player: My character doesn't have the brains to not start this fight, but even if we roll lucky and win this, it would feel broken. Can I roll an attack and then immediately concede?

GM: Sure. What would that look like?

Player: What if my character is disarmed somehow?

Etc.

I've seen where a few outcomes get discussed, and if the group doesn't have a strong favorite, we just ranked them in order of luck, and then determined the full encounter with a quick single roll.