Themes are great because they are very visual. People can see right away if it's right for them.
Why have a WordPress Theme Demo Site?
People like to try things. It is way better to give enough information to the users so they know if a theme would work for them or not otherwise they may request a refund.
You have create an awesome theme and want to show it to your potential clients?
You can enter any content you'd like in the demo site template. You can upload photos or import your theme's demo content. That way users can start testing the demo right away.