Just because they can do X doesn’t mean you shouldn’t protect against Y.
Just as an example scenario, say border guards took my laptop out of my eyesight. A camera or USB keylogger won’t do anything in that case. Hijacking my bootloader though potentially gives them access to my machine without me having any clue.
Secure Boot is useful and worth setting up. But everyone has to decide their own level of comfort when it comes to security.