The egr reduces combustion temperature to control emissions at anything over idle. If it burns too hot, you get NOx (oxides of nitrogen).
it does this by introducing exhaust gas through a passage on the intake manifold. the amount allowed in is determined by the computer.
Since exhaust gas doesn't burn as easily, it pretty much makes the engine run like "controlled crap" not noticeable because the computer knows what its doing.
Take that away and it could burn up the engine soon enough IF not reprogrammed.