Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Can someone enlighten me on the signif

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Can someone enlighten me on the significance of this piece of text? Does it mean you're not allowed to edit the license and publish it under the same name, or does it effectively copyright the entire license text? I'm gravitating towards the former since this is a case where the text was edited but renamed, but I'd still like some clarity. As an aside, is it even possible to copyright a license? There's a copyright notice so isn't this technically plagiarism?