I will second "because it is there". You see a documented option/feature and you start envisioning ways that it can help you craft things.
The first few JQM web apps I did were single page. All of my apps are very dynamic and data driven. I develop with MVC so my views (pages) are interacting with my model. I saw the multi-page template as a good tool for displaying a dynamically injected dialog to obtain data for a field or fields in my model. The dialog contains a form that is handled by the same controller logic as the remainder of the page. Simplifies things.
I've also thought it would be good for creating dynamic "pages". For instance I have a TV Network mobile site. There are different shows on the network. Each show has different episodes. I thought that I could use the multi-page template to inject dynamic pages for each episode in order to provide a carousel of sorts that allows a user to thumb through the episodes.
I am discovering that the side effects of the multi-page template at this time make it more of a pain than it is worth. But you asked about why, so there is my why.