Superman can't be every where at once. It's why they have multiple fire trucks at a station cause what if you have TWO fires. Captain America is Stronger then Both Black Widow and Hawkeye yet both are still on the team with him because they bring different skills to the Battle.
Matoko said it best: "If we all reacted the same way, we'd be predictable, and there's always more than one way to view a situation. What's true for the group is also true for the individual. It's simple: overspecialize, and you breed in weakness. It's slow death."
Basically it's Good to have overlap, Supes is the Heavy Hitter. He's the guy you sub in when the Bases are loaded and you need that homer, let your heavy hitter heavy hit.