Its just a balance between performance and budget. After a few runs (maybe less depending on the surface and the compound) they will start to lose performance. Most people run them until the performance hinders their abilities. If you crash a lot then new tires won't make much of a difference. If you are really fast and .1 seconds per lap makes a difference in the result you might replace them every 1-2 runs. Obviously once the belt shows you are at the end of life.