I've never used it. Here's a better explanation of what it is used for.
The design goals for Windows XP on a typical consumer PC are:
Boot to a useable state in a total of 30 seconds
Resume from Hibernate (S4) in a total of 20 seconds
Resume from Standby (S3) in a total of 5 seconds
Boot and resume times are measured from the time the power switch is pressed to being able to start a program from a desktop shortcut.
http://www.windows-help.net/WindowsXP/tune-19.html