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.
Achievements (for me, at least) are just a reason to spend more time with a game that I enjoy. In most cases, I have trouble enjoying a game if I don’t have goals to work towards (either game-imposed or self-imposed). If I finish the main part of the game, and am not tired of it yet, achievements give me goals that I can follow if I want to keep playing.
Definitely agree that there’s too many games that have achievements that are just in no way worth the time and aren’t even fun as an auxiliary goal, though. The best ones are the ones that get you to do things you otherwise wouldn’t (e.g. playing a non-standard playthrough of the game). The lazy ones (‘Kill X enemies, Earn Y dollars’) are just busywork or earned ‘automatically’ while doing other things and add nothing.