It sometimes makes sense to boot without a hard drive. This can be done by PXE booting. The motherboard / NIC has to support it and it is not exactly common, but it allows you to manage desktop OS installs like VM images. Very flexible and robust. I don't use it to manage 10 boxes but managing 20+ as some people do here, I could see a need for something like that.

One of the better PXE solutions out there is a company called Ardence, which was acquired by Citrix.

Here is a great video of them showing off their software on around 250 some odd computers.

http://www.youtube.com/watch?v=moIuHqIc-PQ

Their software, as usual, is not cheap. It is designed for the enterprise IT setting. There are other methods out there - Red Hat has something like this built in to their kernal and there are several other vendors out there that will enable this.