But we have a couple of good ideas. Professor Kevin McGowan is Associate Curator of birds and mammals at Cornell University. It is protection from predators by being in large numbers. By being in these huge groups you may overwhelm any predators that might try to take crows. Great Horned Owls being the most notable.
Crows often roost in large numbers.
In Nebraska and Oklahoma roosts have been documented as having up to one million individual crows. There's another idea that they get into these roosts as a place to exchange information. So, an idea of information transfer there. There's also the possibility that there are social interactions that go on in these groups that are important for these birds to figure out where their place is in society. How good a competitor they look like they're going to be.