UEFI secure boot kernel restrictions

Yeah okay, in that case secure boot is as useful as making a sd-card where the write-protect physical switch actually physically prevents writing to the card. I can't imagine that is difficult to do.

No changes to hardware or software beyond the SD-card itself needed.

So what is the point of secure boot ? Why make something so simple (and so useless) so complicated ? What's the point of the crypto and the checksums and all the mumble-jumble ?