If you have multiple user accounts on your computer you might find it annoying to have to click on the icon for your username each time you start up the computer. To remedy this problem you can remove one of the user accounts, or you can just hide it with a registry hack. Continued @ The How To Geek
If you aren't the only actual user on the system, then you can't really hide the other user accounts, although you could set one of your accounts up to logon automatically and then use the Switch User function only when necessary.
This tweak is most useful for when you have extra user accounts setup for file sharing or other purposes, or when there are residual accounts you want to get rid of.