No questions, no viva, no PhD. That’s fundamental, but there are many reasons for particular questions in the viva.
Your examiners might ask you that question because they need you to make something clearer. Maybe there is a typo and they want to check what you meant. A question may be exploratory, there’s something interesting to discuss. They might want to check a detail is correct or that you understand something.
A question might be asked because they think something is wrong; they’re asking to give you a chance to explain it more. A question could be an invitation, a way to start a conversation. A question could be used to change topic or pause. A question could be an opportunity for you to get excited and talk about something you love.
Particular questions have reasons. You might not be able to see those reasons in the moment, of course, but they are there. Your examiners use them to drive the discussion, to help you speak, to address their concerns and demonstrate that you did the work.
Questions give you the chance to show your talent.