Purportedly, this works better than filtering by tags.

I'm not sure that this level of filtering is quite necessary, but I think that the community fragmentation is useful: People go to each site (SO, SE, SU, Programmers, etc) with a specific purpose, and the topics on each site are broken up by tags. Trying to filter both purpose and topic on one site would be problematic.

(Also, the Unix/AskUbuntu/SuperUser debacle is a special case, IMO)