You read something and read it again, go back and reread it, you have some trouble understanding it, or maybe you have some thoughts you want to pursue.
A smart reader (browser extension, e-book function, etc.) that suggests more information based on your behavior while reading. For instance, if you read at a certain pace, and you stay longer on a paragraph, you might get a pop up asking if you need to dig deeper into details or would like to expand by getting related information.
Wikipedia is usually – as its name suggests – a quick look at a subject matter. What if you need a deeper dive into the subject?
A wiki dedicated to deeper knowledge. It follows the working of Wikipedia, but for a subject written in collaboration to be permanently published, it has to have a certain level of deeper knowledge and approved by an editor.