Railways have lots of purposes, only one of which is to do that. The purpose of VTWC is to do that. The purpose of a Tring stopper, for example, is to take a load of commuters to and from London as efficiently as possible. Speed is of relatively low relevance, and if the line to Tring was built specifically for that purpose and not just because Tring happened to be on the way to Birmingham, you can be sure it'd have been a low-speed double-track line a bit more like the Chiltern or indeed the vast majority of other lines around London.
The purpose of the local lines around Manchester, which the lines via Atherton and Westhoughton to Wigan certainly are, is to shift as many people as possible into and out of Manchester in order to reduce the number of people in cars. The fast service from Wigan to Manchester runs via Chat Moss, which is more suitable due to there being only two intermediate stations, one of which is also served by the far more popular (despite being rather slower) tram.
The purpose of buses is, in an ideal world, to take people to their local railhead. For fairly obvious reasons, bussing people around central Manchester is not quite as viable. Yes, there's the Centrelink or whatever it's called, but if every rail passenger going to Manchester city centre started piling on, it wouldn't even nearly cope. That exists more as a facility for those for whom walking is more difficult.