From the link you posted:

    The HTML <dialog> element is used to create both modal and non-modal dialog boxes. Modal dialog boxes interrupt interaction with the rest of the page being inert, while non-modal dialog boxes allow interaction with the rest of the page.

The modal dialogs are easier for the programmer to use but they're quite inconvenient because generally, they ask you a question you can answer only by looking at/interacting with the rest of the page. Which you can't do (the modal dialogs on web also almost never can be dragged around).