The EGR manifold has passages for exhaust gases to recirculate through the engine. There is a "trap door" in the manifold that opens when recirculation is to take place. The trap door is usually a vaccuum diaphragm operated device. It requires a control system to operate. Unless you are copying a specific model car and want to keep it fully functioning as an original factory car then you want a non-EGR manifold. You want EGR only if you've got one now.