As for role, once again I'd like to bring up the Sturmgeschutz III comparisons. Originally designed solely as an infantry support assault gun, the Ausf F version, with a long-barreled 75mm gun, was soon found to be a valuable AT asset and increasingly served in that role as German MBT production lagged behind that of the Soviets. By the later stages of the war, they were pretty much being used as de facto tanks.
I see the LAV-75/M20 intended to fill a similar role to the Stug III Ausf F: infantry support, SP AT gun, and, in a pinch, MBT.
From what the V1.0 U.S. Vehi Guide mentions about it, it sounds like the LAV-75 was originally designed primarily to provide direct fire support to light and airborne/airmobile infantry units and provide them with some organic mobile AT capabilities above and beyond their usual Jeep and Humvee-mounted TOWs and man-portable ATGMs.
As for ammo carrying capacity, I would think that a vehicle designed to serve behind enemy lines (82nd Airborne) or with light or motorized infantry divisions- all of which don't have quite as extensive a dedicated logistical infrastructure as a Mech or Armored division- would need to carry plenty of its own ammo into battle or risk running out at a very bad time. 36 sure is a lot better than 18. By my calculations, it's TWICE the ammo!
