It's not about the narrative or RPG aspect, it's about the fact that the game is fun to play and you want to continue playing. You beat the game, your team is the most powerful it's ever been, and now.... there's nothing left to fight, no other content you can use those characters you've just spent hours and hours building and playing! The point of NG+ is to continue playing the characters that you've built up and going through the areas you've been through before but with harder enemies to test your mettle against. The game locks you out of Act 1 once it is beaten, I'm sure I'm not the only one who would think it would be fun to go back with higher level characters, with more abilities, and replay Act 1 with enemies scaled up to be a challenge. It doesn't matter that it doesn't fit the Role playing story. It's nice to have the ability to continue playing after the story is complete.
