Why does Windows XP require logon authentication?

Is it on a network?

If it is not a secured network, keep in mind that the Windows NTFS file system is a secured file system that applies security and ownership to files and folders. This must be accomplished with user accounts. Hence the requirement for a logon.

In some households, parents may find this useful to control access for their children - like snooping around the checking account or the Internet. Others want to secure their computer from room mates as well.