I'm guessing they can see what fraction of their visitors also visiting their competitor's site.
I'm not sure what else they can gather but I'd imagine that if you can pick up on what "videos/pages/articles" those visitors are visiting, you can seed your own site with similar content to further entice them to use your site as a primary resource and decrease the chance they will visit the competition.
I'm not sure what else they can gather but I'd imagine that if you can pick up on what "videos/pages/articles" those visitors are visiting, you can seed your own site with similar content to further entice them to use your site as a primary resource and decrease the chance they will visit the competition.