Modern engines recirculate some of their exhaust gases back into the engine with the aim of reducing emissions. This is the main benefit of the EGR system. The downside is that exhaust fumes containing carbon deposits are constantly being recirculate through the engine through time causing a build-up of carbon deposits in the intake manifold etc.
These deposits sometimes contribute to the failure of the EGR system itself. Replacement EGR valves can be expensive and in the case of high mileage engines the continual carbon builds up can eventually lead to total engine failure.