I suspect it helps avoid being eaten. Anything trying to eat them is presented with too many bright spots to track, then nothing to see.

Wouldn't that also be true of normal fireflies? It's pretty hard to track a single firefly if there's a bunch of them in the area.

Not so much, there's no competition, so it's easier to track. I used to collect freflies in a jar as a kid. Seems like a similar idea to schools of fish, herds of antelope, etc. Stay together (even if defenseless) and predators have a hard time picking an individual to hunt.

Ah, the paradox of choice (Firefly Edition).