The short answer is everybody, or
everybody managing a server. Here are some applications:
- Hosting: Complete general purpose hosting (Running many independent
servers in one box).
- Experimentation: You are toying with a new services
and do not want to impact the production services on
the same machine.
- Education: Each student has its own server with root
- Personal security box: Run un-trusted applications with
complete control over their interaction with the rest
of the computer and the network.
- Managing several "versions" of the same server/project
and turning on/off each version independantly.
Just think about all the viruses and worms
out there, you end up with a big everybody using a computer
needs this. :-)