Single Page Application is a new and very useful web trend. Using them allows you to build fast and efficient applications and reusable components that are great for mobile and responsive web applications.
Part of the popularity of the Single Page Application is due to its efficiency in front-loading all of the used libraries on the initial load into the DOM and then loading new content as needed. This way, when you want to load a new page, you do not require your user to pull down your CSS or JavaScript file over and over for any new page. You can instead just load the new content, templates, scripts, or data, as you need it. This is a very useful way to build web applications and it makes sense that people want to build them.