He could very well already have all the answers and still ask the questions in order to see if the witness perjurs himself, at which point he would have leverage over him. He could also have most of the information in place and be seeking final validation from a third witness. In both cases, he would have existing information to bounce the 3rd witness's testimony off of. Its highly unlikely Mueller is going into this so late without existing information that he has already received from other witnesses, voice intercepts, or electronic intercepts (emails, cell phone imaging etc).