The purpose of Set abstractions is to provide set operations like union, intersection and complement, which Dictionary doesn't provide. Set operations are regularly needed in various algorithms.

Void vs. Bool issue might be due to fact that Nate Cook's implementation was written at the time of Xcode and Swift beta and there were a lot of type inference bugs. Maybe Void dictionaries didn't work correctly. For example, using map that returned void crashed the compiler in one beta version. This was actually used in Nate's set implementation and I had to modify it to work.