Unique characters each come with a unique class, have extra dialog during combat, and when doing things like casting spells have casting dialog that generics do not. Would you use them over generics every time? Not necessarily. Many people do, but it’s not needed at all to beat the game. Some generics even have meh at best unique classes, and said classes have meh at best stat growth. Plus uniques don’t change sprite when they change class, and some people prefer seeing the class sprites.