Interrogation droids were used as torture devices in coercive interrogation. Various varieties existed, equipped with various implements of torture. Sometimes, they were modified medical droids. While they were technically illegal, Darth Vader still used one during the interrogation of Princess Leia Organa in 0 BBY.
The use of a droid rather than a biological interrogator could increase the fear in the subject, which was usually the goal of torture in the context of interrogation. The alien and mechanical quality of a droid could make it more frightening than an organic torturer. A droid was also more capable of finely judging a subject's reactions to torture, and of applying various "treatments" precisely.
From the standpoint of the interrogators, use of a droid was sometimes less troubling than performing the torture oneself. If the interrogator wished to resume a bargaining tone with the subject after the coercive phase, the use of a droid also helped redirect the subject's anger towards the droid, rather than the interrogator.