Companies are personalizing web sites for us. Facebook only shows us things like what we have before clicked. Google gives us search results tailored either to our user id or a number of factors.
Basically, our perspective of what is on the Internet could be highly flawed due to actions we had no idea was judging us. So usually clicking on Democratic news items filters out the Conservative perspective which helps us be more balanced in our thinking.
If the above video does not work, then try Beware online “filter bubbles”.