Benzotriazole is what is known as a restrainer. In its action it is similar to potassium bromide. It suppresses development of active centers which would result in fog. When used in excessive amounts active centers associated with an image are also suppressed resulting in a general loss of film speed.
Organic restrainers are often called anti-foggants while inorganic ones like potassium bromide are just called restrainers. Developing agents differ in their ability to distinguish between image centers and fog centers. A developing agent like sodium dithionite has little ability to distinguish between these two centers. This is in contrast to the developing agents which are most often used like Metol. Therefore a restrainer may be required for a particular developer while not for another.