When released into the atmosphere, nitrous oxide reacts with oxygen and becomes nitrogen dioxide. The latter in turn becomes smog when it comes into contact with hydrocarbons.
The EGR valve first appeared in automobiles in 1972 to counter this phenomenon. EGR valves basically do this by sending some of the exhaust gas through the intake manifold back into the cylinders. Because exhaust gas most often doesn't burn, it stays and takes up space in the combustion chamber and lowers the temperature there. Older vehicles used to have mechanical engine EGR valves, but the newly manufactured ones have electronic EGR valves.
However, the function of EGR valves remains the same, whether they be mechanical or electronic. An EGR system usually doesn't require regular maintenance, but it should nevertheless be checked so as to ensure that you are always complying with emission standards. A malfunctioning EGR valve could also damage your engine if not repaired right away.