Okay, they already told you that and it doesn't help. Flashback to pictures of cars from an era where they would puke black smoke from the tailpipe. Standards were set on car manufacturers to reduce that...a lot. So the first and most obvious plan was to pull some of that back through the engine and burn it again. Unfortunately, not all of the fuel that goes into your engine gets burnt. Some of your precious dollars were flying out of the tailpipe. By burning the same air a couple times, more material was burnt up and fuel economy increased. Both are big bonuses.
The problem is, already burnt and sooty air is pretty dirty. These systems get filled with this soot and before long the valves and tubes get clogged or won't work. The car tells the valves to open at certain times and it just sticks because its all...well, sticky. This valve is called an 'EGR valve'. This is the most common point of failure.
As engine efficiencies increase through new technologies, less waste flows through the exhaust air. Newer systems tend to foul out less often, but ultimately, burnt air is dirty and will cause problems.
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