Ammo Crates at Random Places

Is there a reason why I can find ammo crates at random places in this game? Didn't the devs mention once that the whole ammo thing would be explained in the game? I am still early in the game so if there is a reason use spoiler tags. It's just weird going into Remnant vaults to find Ammo crates dispersed throughout. It's not like Remnants use Ammo, so where are we finding this stuff? Is it just a gameplay thing? So many questions, I just find it a bit annoying. Thanks for the responses