The onside kick is an historical part of the game. It was originally conceived of as a play from scrimmage where you would punt the ball down field and it was a live ball when it hit the ground. It was designed to open the game because a lot of people were literally getting killed just running into the line every play. It was introduced in 1906 with another new idea to open up the game - the forward pass. Though back then if the forward pass was not completed the defense took possession.