This mod is used for bikes with aftermarket exhausts.
The PAIR valve allows air to pass into the exhaust so that any unburnt fuel gets ignited in the exhaust system instead of being let into the atmosphere. Consequently, with aftermarket exhausts, this can cause annoying popping, crackling and after fires. Plugging the valve prevents the air from entering into the exhaust and thus eliminates or lessens the amount of popping, crackling and after fires.
I installed a TwoBros exhaust and plugging the PAIR valve smoothed out the idle and reduced the loud crackling and popping. I'll still get the occasional after fire under hard deceleration but it's less than before plugging the valve.
You can find more info here: