Those sidings are used to stable the loco off the sleeper service as well as oil unloading. Then there is the passing sleeper service, the local class 156 dmus to Glasgow and Mallaig and the steam hauled Jacobite, and the occasional departmental//engineering/works trains. Prior to the dmus, services were in the hands of 37/4s. Before that it was 37/0s and before that it was class 27s. With a little modellers license you could include the inwards and outwards freight that runs to the Alcan factory at Fort William too and the paper factory at Corpach. I only really know about the diesel era services but depending on the era you plan to run, Fort William, over the years, has seen classes 08/20/21/24/25/26/27/29/37/66/67/73 on a regular basis and has had visits occasionally from more exotic traction like classes 33/50/55.
Observation cars were feature at one time. There was fish traffic from Mallaig in refrigerator vans. Mixed trains, that include both passenger and freight stock were still running in the early 80s. It's always been a popular destination for excursion traffic with all the varieties of rolling stock that that brings. I would go as far to say it's one of the more interesting and varied locations on the network in terms of locomotive and stock variety.