Roles Define What a Thing Is
Tell assistive tech an element is a tab, alert, or dialog.
A Role Is an Identity
A role tells assistive tech the kind of thing an element is: a button, a tab, an alert, a dialog. It answers what is this?
Native Elements Have Roles Already
Most HTML tags carry an implicit role built in. A nav is a navigation, an h1 is a heading, a button is a button.
<nav>...</nav>
<!-- implicit role: navigation -->