The Steelers had great defenses for a long time with guys like Joel steed and Casey Hampton, great run stoppers and hole vloggers that were at least good enough to occupy the line while the linebackers went nuts. They had an Aaron Smith here and there but their off man front was to take up blockers and stop the run, a guy like Hampton who could penetrate just made them better. A 4 man front is when you need to penetrate more and get after the QB because you dont have 4 linebackers to do it with, that's why guys like Suggs and them are either 3-4 linebackers or 4-3 ends