This is always a great way to increase replayability as a game. Allow someone to take their now super-powered characters and explore the previous sections of the game in a new way.
This introduces the ability for speed runs, power levelling, and just straight up fun and enjoyment as you 1-shot kill the bosses you struggled with previously.
Dead Rising does this great by having not only your character retain between play-throughs, but allowing you to unlock ridiculous weaponry in those playthroughs to make even further replays even more "fun"
Megaman blaster..yeah. You had to earn it but it took the game to a whole new level.