And quickly and efficiently transfer that heat into the surrounding air. A tightly cowled wheel pant will impede that process.
Before everyone goes and pulls their wheel pants off... know that it's not by much.
Consider how long you're actually braking hard. The differential temperature of your brake disc and increasing heat rate is so significant over such a short period that the air cooling reduction by the pants is providing, while not insignificant, is not worth any effort to mitigate. Even more so, your wheel pants are open a the bottom, even on tight fitting pants, so as the air heats up, it expands out the opening minimizing the increase in temperature of the air.
Now, if you were driving long distances and braking with wheel pants, then the cooling would be a more significant factor.
For an easy experiment, next time you land, get out right after landing and put your hand on the wheel pants and note any warmth above ambient comparing the temp of your nose wheel pant to the mains. If you really want to be accurate, use a laser thermometer.
In the real world, I've not found any braking response difference between when wheel pants are on or off.
BTW, I have the Matco WI600XLT's and highly recommend them.