6 men on line penalty

[jdl22]

What is with the illegal formation penalty? Why is it bad to only have 6 on the line of scrimmage? Is there some advantage to having 5 guys in the backfield? Never made sense.

[pudley4]

The rule is based on defining who the eligible receivers are on the play. Anyone in the backfield is eligible, along with the player on each end of the line of scrimmage. If only 6 players are on the line, that means that one of the offensive lineman is on the end, thus he'd appear to be eligible when he actually isn't.