Hi, Christian.
Landmarks are essentially a way for non-sighted individuals to navigate websites. Actually, screen readers employ them to help users move between different sections.
You may have noticed that the landmarks tags are pretty self-explanatory: a header is the "heading" part of the page, with the website name, logo, and so on that need to be the same for every website page; the nav is the navigation menu you can use to navigate to the different content provided by the website; the main tag refers to the main section of your page; the footer is the opposite of the header and is placed at the bottom of the screen and, like the header, is the same for every page and contains privacy policies, credit info, and so on.
So, basically, the landmarks' role is to inform screen reader users about which content is located where.