My good reason to carry a knife is that God gave me rather weak teeth and rudimentary claws in an evolutionary trade-off. The hairy-armed person who figured out how to put an edge on a suitable rock made it possible for us to be recognizably human in the first place.

I wear a wristwatch whether or not I have an appointment to keep, and I keep a pen and/or pencil because I am a literate person whether or not I have a specific writing task ahead of me, and I carry a knife because I am a human and not an ape.

A knife comes in handy for all sorts of random tasks that involve separating matter. Like cutting a string, or making a sandwich, or opening a package. It can also come in handy in an emergency, which need not involve a human assailant, and emergencies are by their nature unforeseen, so one should carry a knife all the time.

And in a perfect world where nobody needed a weapon, I’d probably carry a slightly larger knife, because it wouldn’t scare people.