@scarlet I don't think that it's tamper protection, they aren't signed like packages. It's just an integrity check to make sure that the image downloaded completely and that it's same as on the other side. Archives have checksums in them, images don't, burning incompletely downloaded image would be rather frustrating.
Made sense back in the days when FTP was used to download them, with modern protocols that can resume transfer and now that only few use optical media, it's just a tradition IMO.