The User add-on's primary purpose is to bring EE member template functionality outside of those restricted templates, and into regular EE templates. Imagine implementing very customizable and flexible member templates within minutes! Easily display other features in a users profile page such as their recent channel entries, photos, EE comments, Solspace Favorites add-on favorites, and more!
This powerful and very flexible module is also capable of much more. It brings to the table a robust set of front end member management tools and adds much more flexibility to the way your member management behaves. Send an automated custom welcome email to users once their registration is complete, Register new members via the front end, set the email address to be the username for logging into your site, Custom error pages... need we say more?!
Freedom is a good thing.
User also contains a special feature that allows you to assign multiple authors to a channel entry. This is done by using an additional tab in the Publish page of the ExpressionEngine CP. The admin can then select members from the member list to assign them to the entry as authors.