This blog article talks about how to use mutt with gmail. Very handy to have. My would someone use mutt when they could go over to the gmail web interface? Well sometimes you aren't at a web browser, or with me, I want to be able to customize various aspects of the reply that I don't have in the web client.
For example, mutt lets you do things like have multiple identities when you are replying to things, sometimes you can have mulitple signatures and some other features.
And heck, I really like mutt.