The main thing "did the candidate have any questions for us yes/no?" tells you is "has the candidate read the usual 'how to prepare for interviews' guides where

The main thing "did the candidate have any questions for us yes/no?" tells you is "has the candidate read the usual 'how to prepare for interviews' guides where this is noted as a very common question for which the candidate should spend a little time in advance thinking of one or two good questions to ask". I guess that's a little bit of signal about their ability to prep for interviews but you probably are getting enough "does candidate know how to play the job-interview game" signal already...