Get this: what they are for is to promote swirl.
What that means is basically the level of turbulence in the mixture as it enters the combustion chamber, the swirlier it is the better mixed it is which is GR12 for emissions.
I presume a man with a big computer does some gas flow analysis and determines the optimum opening size for the inlet to swirl things up a treat (primarily at the points on the emission test cycle lol) and you fit a flap and bingo you pass emissions.
Generally people do not report any change in power or drivability from ditching them, but around 0.0001 kittens will die per kilometer driven as a result of the increased CO emissions.
Having the bits gouge grooves in the cylinder wall also results in increased co emissions.