Look at F1 cars with Carbon Brakes - they need to get some temperature in them for them to work at their best. Airbus brakes regularly exceed 300C on short runways such as ABZ
We turn the fans only if The temps are exceeding 300c and 5 mins have passed - to allow for thermal equalisation across the entire brake assembly. and Avoid oxidation of brake surface hot spots.
OR if the temps are likely to exceed 500C. Again this would be done before pulling onto stand because you can get quite a cloud of brake dust sometimes, which may cover your groundstaff- they would then be crying off to go home and get changed. We routinley turn the fans on in the after starts whenever the temps exceed 100C and off just as we line up. You must have less than 150c fans on or 300C fans off for take off.
‘The use of autobrake LOW for normal operations helps to reduce brake wear by reducing the number of brake applications to one, and by increasing the brake temperature to moderate levels. The use of autobrake MED is recommended to reduce runway occupancy, when landing on a short or contaminated runway and when operating in LVPs.’
Carbon brake wear is related to the number of brake applications and to the temperature. hence getting the fans on