I've only ever seen that multiple heavy duty pulley setup used for a few things.
The through drive, to take rear engine output forward to the snow blower on my MH-Ford Sno-Go, plus various compressor and generator drives. You would use that for something that operated at speed, like a generator or compressor, more than for sheer power throughput like a hydraulic pump.
If I had to guess I'd suggest your truck was something like an airfield defence floodlight truck. You'd park it and use that belt drive to run a big generator to power the airfield floodlights. I'm sure you can put the transfer case in neutral and just spin those pulleys, as even I cant think of anything you could use that drive for if the axles had to be in drive too.
39 TD20 Pickup
40 VC1 Command
40 VC3 Pickup
41 WD21 Panel
41 WC36 Carryall
41 Highway Trailer K-38
42 Ben Hur
44 Iron Fireman T-36 Snow Tractor
44/50 Saginaw M19 ski trailer x 4
53 Tucker Sno-Cat
86 Nolan Road/Rail trailer