A tire's tread becomes harder with each heat cycle, and even new tires of the same compound can vary greatly in hardness depending on time since manufacture or storage conditions.
If a tire is too soft for the track or stage it will overheat and may not last the entire race, but if it is too hard it will not reach optimum temperature and traction will be sacrificed.
The Intercomp Tire Durometer allows you to make the correct tire selections.