IE's looks aren't great. Actually, most people get so used to IE that they don't want to migrate to some better browser. If they can customize FF to look and feel exactly like IE then its a great way for IE users to migrate to a more secure browser. If you share your PC with others who just can't stop using IE no matter what (there are plenty of such people), then you can change FF's looks into IE and no one will know that they are actually using FF. That way you keep your PC safe from malwares without having to ask your family members to stop using IE.
Cool,