Levels reward player achievement, and the higher your level the more powerful your character is. That goes for all classes without exception, though all characters of the same level do not have the same power. Low level magic, in many cases in combat, exploration, and social interaction, will be more powerful than high level might, and I could give an example of each, but there are also plenty of cases where high level might is much more powerful than low level magic. Strictly speaking, within combat, might is a shade or two weaker than magic of the same character level. I'd say a four level gap exists. So a 5th level fighter was about as powerful as a 1st level wizard.
These observations come from AD&D, not 5th Edition, though. In the new game, the gap is narrower, down to one or two levels I'd say. Magic still remains much more powerful for exploration and social interaction, of course.
I only want these comparative advantages for magic-focused characters because I believe magic is much more powerful than might. To play otherwise would lower the verisimilitude of the game.