XPages are a new design element within Domino Designer on Eclipse 8.5.  This design element is a new way to display data in a Domino web app, and it separates the design of the page from the data within your app.  Thus, you can bind various data sources to your XPage.  Multiple existing forms and views can be bound to the page, and you can then use that data on the page in question.

Plus, surfacing a view through an XPage allows you to make it look a lot more modern.  The difference between XPages and the traditional default view can be seen when you look at tabbed tables, as it allows you to see contents of each tab without the reload of a page.  This can make your app seem to run a lot better to end-users.