The EGR Valve also reintroduces a larger portion of gases during the engine's first few moments running, when a necessarily rich mixture is used to get things going. The unburnt fuel in these first few moments of an engine's run can be a large percentage of its polluting output, especially if it's a modern engine in good repair. Redirecting fuel-rich exhaust gases back into the manifold does (at least) two things: one, it burns a greater percentage of the rich fuel mixture, and two, it ensures that warmer air is sent into the cold engine so as to speed up the warmup cycle.