This page describes some of the ways in which a QEMU virtual machine can be used.
Test Linux Upgrade
I have a laptop running Ubuntu Breezy. I'd like to have a look at what's coming in the next release, but I don't want to upgrade my laptop and potentially break it, and I don't want to dedicate a whole machine to running the new release. With QEMU I can run Linux in a virtual machine and take a copy of the disk image file before performing the normal upgrade process. If the upgrade goes well - well enough that the virtual machine starts and runs okay - I remove the backup and keep the newly upgraded Linux distro inside the disk image.
Play Windows Games
I wanted to have a fix of an old DOS or Windows based game which I found would not run under Wine or Cedega within Linux. I installed Windows 98 inside a disk image and updated it with all the latest security patches and browser updates. Once installed I made a copy of the image so that I could install games into it, but keep my virgin Windows 98 install as a backup just in case the operational one gets screwed up. When playing I use CTRL+ALT+F to switch QEMU to full screen mode.