She doesn't. Like everyone else said, she's it's another mode, another character with another playstyle. Richter is the default game, you find Maria and you have an extra mode essentially. Plus, her cutscenes are all different from Richter, giving the player even more replay value besides just using her for the sake of using her new playstyle for a bit. You can start a game, save everyone and beat Dracula with Richter, then start a new game, save Maria and then save the girls and beat Drac with Maria.
While I think Rondo is a bit overrated, Maria is one of the best things about the game. Her control and attacks are so vastly different from Richter that it isn't boring to play the game again with her. And like other said above, she take more damage than Richter, which balances it out a bit.