I just started to use IMAP for my email when I moved it all to my laptop, but still wanted to access mail on my home computer. IMAP allows you to keep your mail on the server there for you don’t have to back up your mail and you can view your mail on all your computers.
Using an internet based email address like Gmail, hotmail or yahoo allowed you to access your mail from any computer, but then you had to be online to compose your email. Gmail allowed you to configure POP mail in your local client so you could download all your mail and view it offline, but it would not synchronize your online with the offline. This is why Gmail adding IMAP is such a nice surprise. Not everyone has IMAP access to Gmail, but it looks like Google is rolling it out to all users gradually.