As long as the application is small and has only a few pages, it is easy to maintain its state and manage the data. But as the application becomes more and more complex, it gets difficult to deal with templates, template data, data associated with a route, and so on.
So, to make managing complex multi-page Ionic applications easy, we use the Ionic router. The Ionic router is the same as the AngularUI router. For more information, refer to https://github.com/angular-ui/ui-router.
From the AngularUI router documentation:
"AngularUI Router is a routing framework for AngularJS, which allows you to organize the parts of your interface into a state machine. Unlike the $route service in the Angular ngRoute module, which is organized around URL routes, UI-Router is organized around states, which may optionally have routes, as well as other behavior, attached."
Note
You can read more about AngularUI router from https://github.com/angular-ui/ui-router/wiki.