i know but i prefer to use thunderbird with pop gmail.
Well, I mean I'm asking why people like Thunderbird.
Gmail is pretty powerful, pulls and pops from other email accounts, and has an offline mode (though it's a little hidden, under the google labs stuff) that allows it to download messages and be used offline.
I guess if you don't use gmail, thunderbird would be nice, but I move to different computers a lot, and it seems easier to keep my stuff synchronized with cloud services than it would be with something like thunderbird.
Then again, I haven't really used an offline email client in forever. I used to use outlook express, and I think I even used one of the earlier versions of thunderbird, but the various free web emails never really played well with email clients so that forced me onto yahoo and hotmail's sites. Then gmail came around and somehow managed to stuff a pretty good email client into a web site.