To be fair, easyJet has a similar policy - if you show up early, you can transfer to an earlier flight for free (assuming there is a seat available). In cities where EZY flies from more than one airport, you can even get an earlier flight from a different airport - still free. The only restriction is that this policy only applies to the return leg of a booking, not the outward leg.
As PAXboy says, this is a win-win for carrier and pax. Strange that it's not more widespread - and even EZY doesn't exactly advertise the option: you have to dig deep in the website to find it.