I'm thinking of enterprise grade networking hardware for example. Their main OS ROM is encrypted and they will refuse to decrypt if the boot process is altered.
There are no real secrets or user data being protected by the encryption. Heck, actual passwords and user configurations can be stored in clear.
It is about protecting against "after hours" production runs. If you, as a vendor, order manufacturing 1000 pieces of widgets, you want to be sure that only those delivered to you are really produced. You don't want more, and then your own product competing against you on the market.
Hence, encryption. Only your devices can run your firmware. All those extras cannot.
Enjoy all the benefits of keeping stuff to yourself I guess.
Bonus: potentially lowers the barrier to entry to electronics fabrication as more engineer hours are spent getting the fabrication equipment/processes better documented/more accessible.
No one wants that though. That'd make too much sense.
Individuals don't have quite as much to benefit from. I don't bother to run SEV on my personal cloud VMs. Mostly because it's more likely for there to be enough software 0-days to not offer much practical protection. If I was trying to do cryptocurrency in the cloud on top of seL4 or something equally verifiable then maybe SEV/SecureBoot would be worth it?
In that case the user isn't trusted, not because of maliciousness, but because of security mistakes, which are quite easy to make. Having attestation to prove the device is not tampered with before letting it connect to the company network is a good idea.
How else can the machine tell who is its rightful owner?