The way I understand it is that adding ARIA roles will create a taxonomy of semantics (called landmarks) on your site that a user can navigate to using keyboard shortcuts (key bindings) defined by screen readers with ARIA support.
For example, instead of jumping through actionable elements like buttons/links, the user can use a different key binding to jump to ARIA landmarks instead.
Semantic tags such as "nav" should be handled by the screen reader client by default (and have a specific key binding to do so - usually "tab"). ARIA roles - in this scenario - just add another level of semantics and, by extension, an alternate method for navigating through the page.