That's Mayfield Siding, and it's used for trains reversing in P13 at Piccadilly.
The FNW/ATW Chester/North Wales service has reversed there for many years, but the turnround time went up from about 25 minutes to 55 minutes several years ago, so they are parked there for longer.
More recently, ATW has been continuing some early/late services to Manchester Airport, where they have 20 minutes or so to turn round.
Since May, most off-peak services go to the Airport, leaving just the peak services to turn round at Mayfield.
It will probably all change when NR builds new platforms 15/16 at Piccadilly.
In the past, some trains have turned round at Longsight, or even Stockport.