Basically it boils down to historical reasons, that the station was built as an amalgam of different companies.
Since then, it's never been rationalised, with the exception of the old platform 1, which became the servicing and cleaning siding for a while, at least until the early 90s.
The station needs the full assortment of platforms because it's relatively unique for large Scottish stations in that there isn't a depot nearby. Bear in mind there is no access from platforms 2 or 3 to Alloa, so you basically have five bays and one through road available for use for stabling on any given night, and there's overnight services through the station using the through roads.
The lack of a depot means that in order to start units at Alloa or Dunblane, you'd need to shuttle them up early morning from Eastfield or Waverley/Millerhill, whereas parking them in the bays and having the station as a driver depot means you save on that. All the bays are used, although not necessarily in passenger service.