The fact that they "will say what you want to hear" places limits on what you can gain from torture, but the limits don't make it impossible to accomplish anything.
The most obvious case is getting a password. (or lock combo or crypto key, or anything else that you can verify) You torture until you get a password that works. Obviously, there should be good reason to believe that the person knows the password.
Another case is where you know many things but not everything. You ask about all that you want to know, including things you already know. The things you already know are the honesty check. Answers to the unknowns are assumed to be dishonest until you start getting correct answers for all the things you already know.
The most obvious case is getting a password. (or lock combo or crypto key, or anything else that you can verify) You torture until you get a password that works. Obviously, there should be good reason to believe that the person knows the password.
Another case is where you know many things but not everything. You ask about all that you want to know, including things you already know. The things you already know are the honesty check. Answers to the unknowns are assumed to be dishonest until you start getting correct answers for all the things you already know.