The expected points added (EPA) statistic was created in order to measure the value of each individual play in a game of football. That means EPA takes into consideration that yards gained are not all created equal by using the down, distance, and field position of the offensive team at the start of the play and then examining it at the end of the play.
For example, a James Conner gain of four yards on a third-and-3 is vastly different than a Conner gain of four yards on a third-and-5. The first example results in the extension of the drive.