I had wondered before if that slime jacket they wear is actually as repellent to fish as it is to human fishermen who accidentally catch them. Now I know.
It's so gross, they use it to hunt smaller burrowed fish. The hagfish will drop a bunch of slime into its burrow, wait for it to choke to death, then pull it out and eat it.
Also, hagfish only cost 1 supply, so you don't have to make a ton of overlords to get a decent number of hagfish on the board. They're one of zerg's best units.