Landmark roles provide navigation mechanism of the page while marking each region of the page. All the screen readers in the market can recognize the regions of the page if the content authors mark them using WAI ARIA landmark roles. All the popular screen readers assigned a specific command to let the users quickly navigate from one region to another. Using landmarks is also one of the techniques to satisfy 2.4.1 bypass blocks.
Except the application role below all other roles are specifically help users to navigate different portions of the page.