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If not satisfied, tomorrow you can ask Google the very questions you asked, just cut and paste 1 Q at a time. Why tomorrow? 'Cause today is his/its birthday, don't put it/him to work...
 
In short, it was put there to meet exhaust regulations, it feeds exhaust gas into the inlet manifold which will eventually fill with black sooty gunge, it causes your fuel consumption to go up and power to go down. By returning some of the gas back into the system it reduces the heat of combustion to reduce oxides of nitrogen.

Get a plate for your engine off ebay, they come with fitting instructions.
 

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