The problem is, you don't really know how well they've performed. In order to do that, you have to compare them with other pitchers. Someone who pitches in Dodger Stadium at night is going to likely have a MUCH lower ERA than someone pitching in Coors. Advanced stats try to put everyone on as level a playing field as possible so we can compare them. Are current advanced stats perfect? No, but they provide us with the best information we have available as fans.