This is where you upsell into ebooks, video courses and paid training. Also if you build an email list you can build partnerships which will lead to a much higher CPM.
yeah, but then your business model is selling ebooks, video courses and paid training, and this just becomes one of a number of avenues to reach your market. Which is all totally fine, of course, but the business is no longer "a website with the 5 answers to the most commonly asked Linux questions" :)