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.

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