Was just researching for a different post and saw the last two posts here. For some reason new posts don't always show up in bold.
Anyway.... You can't burn something that is already burnt, can you?
Exhaust gas is
inert, i.e. has no O2 content. NOx is a result of high combustion temperature. So, in order to lower combustion temp, exhaust gas is introduced into the intake. This keeps combustion chamber pressure up, to keep power up, but the inert gas inhbits combustion just enough to keep the temp under control.
The EGR is only for NOx reduction but if it were allowed to flow at idle it would reduce the O2 level in the chamber. Remember - HC is raw fuel and CO is a product of incomplete combustion from lack of adequate O2. So HC might go up a couple points but CO will climb quite a bit. Then again, if the EGR opens more than just a little bit at idle the engine will usually stall.