What Does EGR Do?
EGR is exhaust gas recirculation. Hot exhaust gas is cooled and recycled into the intake manifold to reduce NOx gases. As a result of this system more particulates are produced and these particulates require a DPF and also contaminate your lubricant. Roughly what was happening on Euro 3 emission systems.
Then DPF was added because the side effect of introducing EGR was more particulates (soot) in the exhaust.
Both of these systems fail often, especially the exhaust gas coolers and DPF mufflers.