It’s the remains of the old Meon Valley bay platform, which was platform 1 historically and reached as far as the station buildings, where the bike store is now. The platforms were renumbered to the current west to east order after the electrification in 1990. I’m not sure when most of it was lifted, it was probably before the mid 70s.
I suspect leaving the stub siding was a way of avoiding unnecessary resignalling, but they’d have probably still needed the points anyway (but I’m not sure if they’d have needed catch or trap points?)
I’ve seen it used for putting defective freight wagons out of the way in the not so distant past. I think it might also allow for locos to run round a train without using the main running lines. But I don’t think it’s really recognised as a platform as such, it’s possibly just an accident that it still exists.