Within a black widow egg sac there may be 300-400 eggs, and once these all hatch that would end up to be far too many spiders competing for food resources in one place. Since black widows are really lousy walkers they prefer to fly to new locations rather than walk across the ground to get there. Another suggested reason for this dispersal habit is that it prevents "inbreeding", which is always a bad idea in Nature. A species maintains its strength by mixing its genetic material among unrelated members of that species, so if 300 black widows all grew up and mated with one another, perhaps over several generations, it could begin to weaken that group's ability to evolve and survive. 

The eggs hatch within the egg sac and the first instar spiders remain there until they have molted once, at which point these second instar spiders force their way out of the egg sac and commence the ballooning. This is normal in natural settings, but for black widows that may be living within a warehouse there may not be the necessary breezes to move them away. Now they may very well be relegated to setting up their own webs in the immediate vicinity of their parent or finding a way to walk away from that point to reduce the competitiveness. Black widows, once the maternal instinct of protecting the egg sac is over with, are just as happy eating each other as they are dining on flies, and if too many other spiders are nearby they may simply be fed upon.