What does "useful" mean here? Useful the way it'd be useful if butter knives could also cut trees down more easily or if hats were also televisions - ie more uses = more useful?

Programming languages aren't for the machine, they're for humans, and humans make mistakes so we need to design the language with that in mind. "Truthiness" is a footgun, it increases the chance you'll write something you did not mean without even realising.