What is uml good for?

Testing of kernel modifications on a virtual machine without hosing your real machine
Creating a virtual lan network to practice network administration skills
Creating a virtual Beowulf cluster to test clustering technology
(Possibly in near future) Stealth mode --- Use Linux for day to day work on a non-Linux box
Running funky old code on a virtual machine
Demonstrating user's worthiness to the o.s.?

Testing of kernel modifications on a virtual machine without hosing your real machine