Why do we need to show welcome message on the application UI?

This was the question I faced today while discussing the product user 
interface with my product manager, which made me realize that I did not 
have a very good answer for it, as I have never tried to find out why and 
kept using it on all my designs as it was seen on other applications too. 
Can someone point to some research or guidelines on why to use the welcome 
message with user name for web based applications?

Sandeep Rathod

        Tip of the Day: Forward out-of-office replies to
                    mailto:[log in to unmask]
     CHI-WEB: www.sigchi.org/web POSTINGS: mailto:[log in to unmask]
              MODERATORS: mailto:[log in to unmask]
       SUBSCRIPTION CHANGES & FAQ:  www.sigchi.org/web/faq.html