The gigabit boot agent is a small piece of software in the BIOS, (on the k8T anyway) that allows you to boot your pc from the network.
If you have a dhcp server properly configured, you can give have it give your machine an IP address, and also tell it that there is a server on the network that has a boot image that it should load. You machine will then connect to that server, retrieve the boot image, and load it.
Most cards that have a boot agent use PXE, which is a Preboot eXecution Environment.
I have used netbooting in a few places that I have worked and it was very useful. I used it at a college to allow any computer connected to the network to be netbooted to a dos based ms-networking prompt, so I could reimage the machine with ghost instead of having to carry around a network boot disk.
You could also use it for kiosks, or so that you could have machine with no local storage.
I was looking into using it to boot some pc's straight into a dos version of citrix, so that some old pc's wouldn't need hard drives at all.