I too am curious why there is no single fare.
I suppose there are very limited circumstances when it would actually make sense to use this ticket. Mainly, when you need to connect between a destination only served from LGW and one only served from LHR. If you do this on separate airline tickets, then you should really allow at least 6 hours to transfer between airports. On a connected ticket, BA allows 3 hours and will rebook you if there are delays on the M25 (as confirmed by National Express).
Otherwise, just pay a bit more to fly from the same airport; it's rarely going to be more than the price of this rail ticket. National Express is £25 each way usually, but of course you can get it down to £4.50 each way with the X26 bus and Southern from East Croydon.
I'm always amused when my friends who live in west/central London, who aren't exactly poor, choose to fly Ryanair from Stansted because "it's so cheap", and also amused by my Irish friends who subject themselves to an LGW-LHR transfer because DUB-LGW//LHR-XXX is 50 euros cheaper than DUB-LHR-XXX...