If you publish a blog, and allow comments, you're going to occasionally get comments that you don't appreciate, on your blog. Some comments will be blatantly offensive, and contain links to hacking activity, porn, and / or spam. I won't provide an example here, but some that I've seen recently have been pretty nasty. If you're going to keep your blog suitable to the general public, you're going to have to moderate comments, either before or after publishing.
If you choose to moderate comments after publishing, you're going to have to check what's been published, regularly. Long ago, if you found a published comment that you didn't like, you'd simply click the trash can icon, and then click "Delete Comment". That would leave a comment entry, with the name of the commenter, time, and date, and a notification "Comment Removed". Any nasty comment content, with links to hacking, porn, or spam, would be gone.
If you decided later that the comment in question deserved to be there, you could come back and reverse the deletion.
Then Blogger started allowing commenters to post anonymously (at your option), and to attach the URL of their choice to their names. Of course, the spammers figured out that they could post anonymously, with a link attached to their name, and pointing to their web site. And when you deleted the offensive comment, and left the commentors name, the link to the spammers web site would be right there for your readers to click on.
So Blogger added the additional option, "Remove forever", which you can check before hitting "Delete Comment". The "Remove forever" option simply erases the entire comment entry, including the commenter's name (with obnoxious URL attached), the time, and date, and comment content. So no more link to the offending web site.
Of course, "Remove forever" comes with the notation "(It can't be undone.)". If you select "Remove forever", you'll never be able to restore the comment. But with hacking, porn, and spam, you won't want to do that, anyway.
So, the next time you get a truly obnoxious comment on your blog, just hit the trash can icon, select "Remove forever", and hit "Delete Comment". See the entire comment vanish, and imagine how disappointed the spammer must feel.