"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 carry 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 unforseen, 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." - James Mattis

A knife and a Bic lighter are the bare minimum pieces of gear I consider one should have. We could get into the ferro rod argument about what is the best way to make a fire, but if you were to look at any office building by the entry door, when it is freezing cold with winds blowing at 40 MPH, you will see a group of people smoking. If there were a better way to light a cigarette, these folks would use it. Just carry a Bic lighter. Plus if you are incapacitated someone else in your party will know how to use a Bic lighter, hell, even a child. They will not know how to use the latest and greatest fire making doo-dad.

But you should know how to make a fire and a suitable edged cutting tool in the event that you are separated form your gear, or know how to get by without. Knowledge based survival.