[cynwoody]

That's true.

However, spam filtering (for sure) and prioritization (maybe) improve the user experience, thus drawing in more clients and retaining the existing base.

Spam filtering, etc., make the service more attractive, boosting the user base, which makes it possible to charge more for the opportunity to show ads to the customer base, which, of course, is how Google gets paid.

Spam filtering, etc., are no different than improving the Search Engine. Both attract more eyeballs and, thus, more ad revenue!