They're pack horses, you find a heavy weapon and your already loaded, you pass it to the companion. Regardless of what it is, Fat Man or missile tube, even if they don't have any ammo for it at the time, they still seem to trip over a shell during a fight and switch out weapons when they run outta bullets, or at least they did in F3 all the time seemed like.
So I'm just hoping that was changed.
Them using medical kits, or healing powder, or anti-venom or stuff like that you put in the inventory I'm hoping was fixed because I cant count how many times I had to drop back to a save file after the companion died in NV or F3 because they were out of my reach during a fight to pop them with stimpacks.