Think of voluntary censorship in terms of other things like voluntarily abstaining from alcohol, drugs, or even meat.

I avoid things that I believe are bad for my body, and I avoid things that I believe are bad for my mind. You might argue that it's impossible to consume something "bad for your mind", but I disagree. I think there are certain forms of entertainment (written, visual, and interactive) that absolutely can desensitize you, distract you, addict you, influence you (negatively), etc. - I try to avoid those effects where possible, and self-censorship is just one tool to combat them.