They could be FBI agents tracking down a ring of domestic terrorists, or CIA agents trying to unravel an international espionage system. They want confessions, but they want a great deal more. They need information. There's a conspiracy out there and they want to know about it. The defendant recognizes that little is gained by simply implicating oneself.
Genuine confession involves contrition and cooperation, and that means naming names.
Thus we find the defendants doing what defendants do in all ages. They provide the requisite information but try to limit disclosure as much as possible, naming if possible only those accomplices they assume are already known to the authorities. As for themselves, they readily admit to less serious actions and to actions about which they suspect the inquisitor already knows, but are less forthcoming about other matters. The inquisitors anticipate all this and have prepared a set of questions designed to prevent evasion. These questions serve as the filter through which what we know of the heretics must pass.