When you have been harassed by an individual or a group of individuals, it might be more clear to you why a person would want this feature. I want to be able to use and enjoy the site without worrying about being harassed by people who can see my posts and stalk me that way. People who choose to view my posts so that they can criticize or harass me.
Just because that's not how THIS forum works doesn't mean that's not how forums work in general. I have been using forums for over a decade now, and this one is very unique in how the ignore feature is carried out. Facebook is a very recent example, but there are many, many other forums that work in the way I'm asking for here.
If you offend someone to the point of them wanting to ignore you, why should it matter if you can see their posts or not? I've never been bothered when somebody ignores me and I can't see their posts.