I've heard that it's to give extra time to a player that last participated in play and therefore exerted themself. That is, the ball hasn't nearly got back to the centre by the time they reach the bench, so it's effectively an extra couple of minutes rest where there is no active play. That means they get back on the ground quicker, missing less action. Doesn't make sense when a forward who has done little running kicks a goal, but overall it works for the team, particulary for midfielders. Interesting to note if this continues the more the interchange cap is reduced, which I think it will. Possibly down to 50 per match over the next 5 years.