Either one or both works.
Mine is completing the Pokedex in the original Pokemon games. All you get is just Professor Oak giving you a wink and a small few second cut scene. And a congratulations text. Imagine spending all of your time then, getting all 151 and even 252 pokemon just for that? Yeah no thanks, I never completed the pokedex.
Going the Joja-Route in Stardew Valley. I say this mainly because, it is what you make of it. You forfeit being able to complete the Community Center by earning things, when you sign your soul away to Joja. What I would’ve liked is seeing Pierre go out of business completely. I just think that would add a route of depth in the game where you have to make ends meet through Joja because Pierre is permanently closed.
But, that doesn’t happen, he’ll still be in business despite his depression about Joja running things. Kindof ruins the whole concept of doing it for the achievements even.
Challenges in action games are worth completing most of the time because they’re typically designed to either drive home the intended purpose of individual combat mechanics, or outright reveal mechanics too advanced to cover by basic tutorials—e.g. dodge counter in Hi-Fi Rush.