Well, as long as you have credintials, any email client will let you use your account on any system I think the main thing is that you can't really add on a lo

Well, as long as you have credintials, any email client will let you use your account on any system I think the main thing is that you can't really add on a lot of things to webmail. Look at the success of outlook in office, because you can add so many things to an exchange server that IMAP doesn't support. Reserve rooms for a meeting, schedule events, and more. Or, small organizations often have a single IMAP server, and do not use gmail. If you are part of several servers(graduate students/scientists/professors), a client allows you to access your mail from all of these systems. If they have webmail at all, it's awful(Think Squirrel Mail). I sort of tried adding my university email to my gmail account, but the feature always broke in weird ways.