I've been led to believe that asteroids in an "active" field are hurled at ships on your escort list. This is not the whole of the story though.
So, let me give you the context. There's a mission in Syrk where you defend science vessels in an asteroid field against a hostile faction. There are two hostile cruisers and some fighters.
I observed that the science vessels fall due to the asteroids rather than the cruisers, so I thought I'd modify the asteroid field's attributes to make the mission easier. I kept replaying the mission, trying out a wide array of attribute settings, but I discovered no change in difficulty. Then I decided to take extreme measures and set the asteroid field's diameter to 300,000 meters and lowered the number of asteroids to 5 (yes, five asteroids overall).
Then I replayed and observed that the engine keeps creating asteroids around the ships on your escort list. It's not just hurling the already existing asteroids at them. This keeps up until said vessels are destroyed, irrespective of any preset values in FRED's Asteroid Editor.
Then I made a new mission with one Levy in a similarly huge and scarce field. The conclusion is the same.
I don't consider this a bug, that's why this is not in Mantis. I don't intend to complain, either. I just felt like sharing this little discovery of mine. Hope there's someone who didn't know this.
