Why so many players on a football team?

Answer 1

This question was posted in both the American Football and Soccer Football categories, and so both will be addressed.

In the NFL, Rule 5, Section 1, Article 1 states:

The game is played by two teams of 11 players each. If a snap, free kick, or fair-catch kick is made while a team has fewer than 11 players on the field of play or the end zone, the ball is in play, and there is no penalty. If a team has more than 11 players on the field of play or the end zone when a snap, free kick, or fair-catch kick is made, the ball is in play, and it is a foul.

By virtue of this rule, only 11 players from each team may be on the field at a time.

In Soccer, the Laws of the Game, Law 3, Section 1 states:

A match is played by two teams, each consisting of not more than eleven players, one of whom is the goalkeeper. A match may not start if either team consists of fewer than seven players

By virtue of this law, only 11 players from each team may be on the field at a time.

Answer 2

to fill all of the positions and have extra for injuries