Because I want a fully clean system to run Logic Mainstage on for live music, and everything needs to run very smoothly. The best way to do this is installing a completely different system that doesn't go online at all, or do anything that's not absolutely necessary for Mainstage use.
This will be much more effective than just creating another user on the same system.
I also have a 200GB 7200rpm in my MacBook, so allocating ~40GB to the Mainstage partition isn't a problem. I also don't want to be tied down to an external HD if I can avoid it 🙂
Thanks for the help guys, it's good to know it can be done.