Speaking from my background, it prevents someone from trying to boot using an external device to access your system, assuming you have a BIOS password in place.
Of course encrypting your drive works just as well, but security in depth demands a “why not both?” Approach
It's supposed to prevent unsigned files from being loaded by the UEFI (AFAIK) which could possibly help with rootkits, if it doesn't somehow sign itself. However, these are pretty rare if you don't allow sketchy software to access your boot partition, and will often cause issues with non major Linux distros.
With Debian I think I was able to load the appropriate keys after installing the OS and then re-enable secureboot in the bios. Might be worth checking into.