The biggest advantage here is that somebody has intellectually made a decision to include or exclude access based on a set of predetermined criteria. And finally, host blocking is reactive rather than proactive and there's no way around it. In a nutshell, that's how it all works. A condensed version to be sure, but hopefully that will give you an idea of what I looked at from a technical perspective. Its popularity is due to a lot of bad press regarding the dangers of the Internet - all those evils lurking behind every click of the mouse.
Parents are concerned, and rightly so.
In less than three clicks I can download some of the most disgusting and dangerous material ever produced by the human race. It doesn't mean nobody can, it means certain individuals can't until specific criteria are met. I don't view that as harmful, but as part of growing up and becoming mature enough or having enough background to handle the responsibility and consequences. The information collected in any library is acquired according to a preset policy that outlines the type of materials that should or should not be gathered. Nothing would cause me more fury than to suddenly have my general ISP tell me what was appropriate for my home, my culture, my life.