Asking why your research is valuable starts the work of exploring what your significant original contribution is.
Asking why you wanted to explore this area gets at a different but equally fascinating discussion: what’s your interest?
Everyone has their reasons for why they do what they do. Those reasons aren’t always near the surface though. Over time they can even become buried beneath the daily hard work involved in doing a doctorate. I loved the challenge of maths – it was intriguing, it was hard, it was puzzling – and by the end of my PhD I was starting to forget that. It took a little re-reading and exploring to uncover that again.
Whatever your situation, reflect on why you do what you do. What’s your interest? What fascinates you? Reflect on the big picture and on each chapter. Pick out what gets you excited and fascinated.
Your examiners will be happy to find a passionate, excited researcher in the viva.