Two uses. Either running your batteries down at the end of a race day to storage levels. Sometimes the built-in discharging can take quite a while, especially if you are limited to around 30 watts of internal discharge capacity. The other thing people use high-amp discharging for is to quickly deplete a battery between races so they can then hit it with a lot of charging amps in an effort to heat the battery up and lower the internal resistance.
I use it as an occasional health monitor of my batteries. Charge them fully and run a high amp discharge program to check IR and voltage levels at 1 minute, 2 minutes, 4 minutes and 8 minutes.