A lot of times in modern games, GMs may want foes with modern military weapons to face the PCs.
The PCs sure as heck want to have access to modern military weapons. Usually extremely oddball special-purpose weapons, but in my experience they won't turn up their noses at grenades, assault rifles, and grenade launchers.
But you can't just pick up some M9, M4, M203, etc., right?
Apparantly if you take the very tiny loss rate of weapons from the US military and multiply it by the millions of weapons they have, it's not actually as rare as you might have thought.
This piece in the Military Times is a bit chilling in some ways in a real-world sense, but in a game sense? This is "that's why the fringe criminal group you're facing all have M4s!" justification.
US Military Guns Keep Vanishing, Some Used in Street Crimes
It reminds me of that scene in Lord of War when the main character tells a Russian general to, basically, declare 10,000 or something AKs lost and sell them to him. This is a much smaller scale, and is more likely a mix of thievery, accidental loss, and bad record keeping . . . but it's a way to put modern military guns into the hands of player characters who aren't in the military proper.