The purpose of this function: with passwords, your authentication belongs only to your credentials, in case they are stolen, anybody can log with your credentials (stoling a password is really easy today, just check tabnagging :
http://www.azarask.in/blog/post/a-new-type-of-phishing-attack/ , click on another tab and wait 5 seconds ^-^ )
Using a browser authentication, any fraudulent access would need to be done on your own browser.