Well...

One real benefit of leveling slowly (while getting your BP higher) is that you'll get stronger against people of your level. For example, in sage world battlefield, I think that your opponents there are of the same level as you. So naturally, you would win if you have a higher BP (base power) with the same level that you have. I had a hard time with my character since my goal was to level it up fast and not to raise my base power. I barely win in sage world battlefields as well as in the fighting matsuri battles.