Microsoft Knowledge Base Article 306541 contains the following summary:
This article describes how to manage stored user names and passwords on a computer that is not a member of a domain.
When you log on to a Windows XP-based computer, you can supply a user name and password, which becomes your default security context for connecting to other computers on networks and over the Internet. However, this user name and password may not provide access to all desired resources. The Stored User Names and Passwords feature provides a way to store additional user names and passwords as a part of your profile.
Stored User Names and Passwords is a secured store for password information. With this feature, you can type user names and passwords for various network resources and applications (such as email) one time, and then have Windows automatically supply that information for subsequent visits to those resources without your intervention.