It automatically marks stuff you've seen - it's just a visual cue. Similar to how search engines (like Google) show visited links with a different color.

I think keeping them on the page instead of automatically hiding them makes more sense for a product that's trying to update their news feed once per day. You feel more in control, as if it's not a stream of never-ending stories, but rather a fixed amount of stories that you can realistically power through. Seeing all items checked sort-of supports this philosophy.