it's a system with a group of people... they often have email and static site stuff which is "public facing" but they also have things like internal bulletin boards to chat with other users on that system, muds (multi user dungeons), sharing files, working on projects with others on the system, being able to discuss and share information, get help doing things, give help to others, etc. these are the benefits and real reasons to use a pubnix. i'd expect something like 97% of pubnix users run some *nix already, they aren't doing it to get first time access to it because it is in practice easier to run a sandbox *nix in browser, or vm as you indicate. If you don't see how being a member of a multi user system is fundamentally different from having a private server, you probably [i]really[i] don't need to be using one.

tl;dr : facebook for *nix autists.