The Simulator can evaluate and compare strategies against various other strategies that opponents might use or how they might perform.
The basic idea is to have a faster turn-around on strategies than always running against the sparring partner, whose availability is limited by DARPA.
For instance, one could model an opponent that always exploits a service after ~10 rounds, always immediately, or never. Similarly, one could model an opponent that always fields replacement challenge sets or never does so.