It allows you to specify custom difficulty settings that are written to thief3's memory every 100ms. Supports Thief deadly shadows 1.0 and 1.1
Apparently ion storm fudged up with thief3 and game difficulty settings get reset to normal when you load a saved game. this obviously makes playing through the game on harder settings slightly difficult. thiefbot is a hacky workaround to be used until a working patch is produced. simply put, it allows you to specify custom difficulty settings that are written to thief3's memory every 100ms.
while thief3 v1.1 fixes the difficulty reset bug, i believe towns are still normal difficulty (i havent tested this, but it would make sense). thiefbot, while not being needed to fix the reset bug, can still be used to set the town difficulty to your personal preference