I must be missing something. What does Secure Boot solve, that I care for, as an end user?
It's mostly useful to those of us who use full disk encryption.
When you turn on disk encryption, the contents of your hard drive are encrypted using an AES key. This key is generated randomly, then encrypted using whatever password you set (contrary to popular belief, the password you set is not the encryption key; it's used to encrypt the encryption key), then recorded on the hard disk itself. That means if someone gets access to your computer, they can get access to the encryption key. They can't make use of it, because the key itself is encrypted, but if you chose a bad password, they might be able to break the key, and then get access to your hard drive.
The secure boot hardware moves the password off the actual disk and into a special high-security chip similar to the secure enclave in an iPhone. As several cases have shown, even a nation-level actor like the FBI or NSA can not extract information from this chip.
You can also set it up so that a hostile actor who has access to your computer can not boot your computer from his hard drive, potentially giving him access to your files (if they're not encrypted). Basically, if you turn on the secure boot option that prevents booting from another disk, a person who steals your computer will have an expensive paperweight.
