The main purpose of the line when it had a regular passenger service was to enable travellers from Stockport and points south to pick up trains to West Yorkshire (and vice versa) without having to transit between Piccadilly and Victoria stations in Manchester. As Piccadilly began to receive more West Yorkshire services this purpose became redundant.
One nugget of historical trivia regarding the Stockport / Stalybridge line is that 40 years ago there was an unadvertised hourly all-night DMU shuttling both ways between Stockport and Guide Bridge - with request stops available at Reddish South.
The 1979 Working Timetable shows regular staff trains running Manchester Picc - Longsight Staff Halt - Stockport (reverse) - Reddish South (on request!) - Guide Bridge, and vice versa through the night.
Departures from Piccadilly were at 0100, 0158, 0258, 0400 and 0500 (plus the first train of the night, the 0017 Stockport to Guide Bridge)
Returning from Guide Bridge at 0032, 0130, 0230, 0330, 0430 and 0531 (last one ran to Longsight Staff Halt only).
A little OT for Stockport/Stalybridge, but in the same 1979 timetable there were also all-night staff trains (a class 506 EMU) between Piccadilly and either Mottram Yard Halt or Hadfield (direct, not via Glossop). These also ran in both directions, 7 nights a week, departing Manchester at 0011, 0108, 0206, 0311, 0411 and 0515. Stops were at Ashburys, Gorton and Guide Bridge.
Presumably these regular-interval night trains were provided largely for BR staff going to & from work in the Guide Bridge, Dewsnap Sidings and Mottram Yard areas, when the Woodhead route was busy with 24 hour freight traffic. And the same staff would have used the public daytime trains when going to/from their shift change or to sign-on/sign-off during more social hours.