One of my projects has been developing some automated install tools so that we can more easily install our software (current install process is several *hours* and is all manual!). Only way to test it (properly) is wipe the machine and run it again...
Wiping only takes ~12 minutes so not the end of the world but obviously not really practical to install extra software on it (and as above - it's not and never will be on the network).
EDIT - For reference, I've improved alot of the installers and added some scripts to call all the right software in a silent mode, and to copy pre-configured preferences in etc. Now takes less than 30 minutes with only a few manual steps (i.e. you can't screw it up!).
EDIT2 - And yes, I'm aware making a single image of a complete install and re-using it would be a lot quicker and easier to setup but it's not a route I wanted to use (difficult to handle upgrades, doesn't work if we change machines etc).