We want our games to present both persistence and mutability in certain degrees and certain forms, but that varies from person to person and time to time. When those factors are not in balance, we can be left thinking, “What’s the point?”
We want the effects of actions to stick around, but not for too long. You want the game to keep track of your score, but it should reset between games. You could completely empty a world where enemies never respawned; you could scarcely progress through one where they all respawned instantly; you could certainly find yourself “done” with either in short order.
In an MMO, character advancement is more persistent, while your effects upon the world are very mutable. Monsters respawn within the minute. The dungeon resets as soon as your group leaves. Your levels and equipment stay with you, and you tend to exploit mutability to farm monsters and dungeons and thereby increase your levels and equipment.
Outside our MMO world, game persistence is largely bound by the unit of a “game.” Little carries over between games beyond a win/loss record. You would not play a “new” game of Monopoly if the previous winner still had all the money and properties s/he ended with. Sports would be very different if winners were determined by cumulative score over an entire season. You reset the world for every game of Civilization, and you reset the story to start over a single-player game. “New game plus” adds more of that between-game character advancement.
Finding the right balance can be hard outside of established norms. Adding a bit of the right kind of persistence is hard, as is mitigating it with mutability. You want your actions to have an impact on the world, but you do not want to be forever bound by others’ actions. You want to be able to reasonably counter other players’ actions, but you do not want them to trivially counter yours, and I think you’ll find that your perceptions of “reasonable” and “trivial” can depend on whether they are your actions.
Sometimes, I feel like fighting that boss again or running that dungeon again. Other times, I really want save the kingdom and have it stay saved after I log off. Luckily, we are not bound to play the same game all the time forever, so we can seek the mix that suits us each best at the moment, but the movement between games is itself a form of mutability.