For example I don't consider myself an especially intelligent person (on the right side of average I hope), however I know people who are very intelligent and can talk at length with authority on a vast number of topics and are seemingly able to absorb and retain information instantly. Also I am exposed to forums like HN which are full of smart people.
So naturally whenever a conversation or thread comes up where I feel that I don't have anything to contribute but I know it sounds fascinating I can't help but to try and learn everything I can about it so I can weigh in next time, which in turn leads to finding gaps in my prerequisite knowledge of the subject (and interesting offshoot subjects). Which of course leads to having 12 tabs open.
This is why I think it is better to pick a few subjects that you decide you will learn in some detail and buy books on the subject.
The great thing about books is that they follow a linear progression and most importantly they end.
The problem with HN is that you can read every article on the front page along with a bunch of wikipedia, but the next day there will just be more.