Imagine instead of an RPG, you are playing a tactical level wargame along the lines of ASL. Suppose you have a series of combat simulations. If in each combat, you only have so many grenades, rounds of ammo, or bazooka rounds, then you still have limited resources. But, if you come into each combat with the same number of grenades, rounds of ammo or bazooka rounds (which may in some cases be realistic) and even soldiers, then the game is ignoring the operational level of play (there is a helicopter which flies in and restocks the platoon after every fire fight). That can be alot of fun. Operational book keeping can be tedious, and it can (as you noted) be hard to design good balanced operational scenarios.
But sometimes its fun to say that you only have a limited store of supplies that 'the helicopter' can bring in, and once they are gone then they are gone (or perhaps that you only recieve resupply at a limited rate). This turns the series of combat simulations into whats known in wargaming as a campaign (or mini-campaign depending on the scale). The game is still just as tactically rich as before, but now you have to balance tactical decisions (throwing this grenade would be helpful right now) against operational considerations (I won't have the grenade later).