What's in a formation?
Football is a game with many arbitrary rules. Why four downs to get 10 yards? Why not five to get 15? Each team can field 11 players. Why not 12 or 13, Tennessee?
Arbitrary rules also dictate the basic structure of offensive formations. Of the 11 on offense, four must be in the backfield, or behind the line of scrimmage. That leaves seven that must be on the line. Obviously, one of the players in the backfield is the passer (who may or may not actually be the quarterback), but the other 3 are eligible to receive a pass. Of the players on the line, each one on each end is also an eligible receiver.