There is a good reason.
We had a thread which went a bit inflammatory, a member posted some unpleasant lies about another member.
After removing the offending post we want to make amends by contacting people and explaining, but if someone didn't see the thread we don't want to stir things by mentioning the situation at all.
So we want to only contact anyone who might have seen the post, hence wanting that info.
I hope that makes sense now.