Because you need to monetize your work, and you need to show potential advertisers the numbers of how many "eyes" the ad will catch.
Discussion
What this guy says - signing up is like paying by proxy (ie with your attention). If you want to support us non-monetarily would be great if you did, otherwise ghost auto generates RSS feeds; you can find ours at https://therage.co/rss
Sorry for the pong... Did you get 4 reactions to this post from me? [just testing some stuff]