On of the reasons that used to be used, was that IE was a problem, because of it's size. There seemed to be more attacks by hackers...much like the IBM computers vs. Apple....size matters, but not always for the good!
FireFox/Mozilla was an alternative which became popular. There are several out there, with one of the newer ones being Chrome (which is Google backed, I believe).
If you are not having problems with IE, stick with it. I still have it available on my computers, but generally use Firefox as my primary and Chrome as the secondary server. I have on a few occasions....and one of which is when I'm on RV.net on my home computer...have to use IE to be able to respond to the different threads posted. I can read them with Firefox, but can't respond for some reason....but it's fine with IE? Go figure??