Often the wasp will search for an empty nest before building her own. Sometimes turns out builder was not dead after all, comes back to contest for nest, though in a few species end up sharing nest, though fight every time they meet. When one female leaves the fight she will probably go and try to take over another nest. End result will be that one nest will have offspring of several females in it. Presumed each one builds own cell.
Advantage is that communal nests are continuously guarded .
This means predators and parasites have a hard time getting in. It also means the wasps are able to dig deeper nests, which can aid in regulating temperature in the desert where they live. Also means mutual applications of protective coatings. Certain parasites have evolved very specialized behaviors which enable them to get in. These parasites specialize in communal nests.