"What You Have On You"
Emergencies have a way of showing up when you're not expecting them.
Somebody swerves to miss a cat and runs you into a ditch. A deer (or elk, or moose) suddenly leaps into your headlights, allowing you 20 whole feet of stopping room at 55 mph. The weatherman lied (again) and here you are in a hailstorm with no jacket. A peaceful afternoon at the lake becomes tense when a swimmer is entangled in someone's fishing line. The weatherman lied (again) and you're stranded by the road in the midst of forty other cars, all playing fender tag, as they try to come to terms with the freezing rain on the road. You come back from the river with your day's catch, to discover that your truck won't start, your cell phone has no signal, and sundown was twenty minutes ago.
And the list goes on.
Survival is, of course, not all about the knife, but it's arguably the most broadly useful tool you can have. A good cutting tool can make all the difference between making it and not.
I have, at all times, a full-sized folder (EKA Swede 92) on my belt in a nondescript horizontal nylon pouch. It's more or less the size of a Buck 110, but a little slimmer and lighter. On the other hip rides a Leatherman Wave. Additionally, there's a small "gent's knife" (Case Pocket Hunter), and -- depending on the day -- a SAK or a stockman.
If I'm dressing up, the hardware will change, and I may be down to a couple of items but, unless I'm in a courtroom or on a plane, I always have at least two.
The truck has a large fixed blade, a large folder, a folding fillet knife, another SAK, a hobo tool, and so on. The Jeep has a knife or three, a hatchet, and a short survival machete (Ontario Spec Plus).
Keeping knives, hatchets, and such in the vehicles is a habit from years of living around forests and lakes and mountains.
The Idaho Driver's Manual actually has a list of stuff they recommend you carry in your car -- including a pocket knife.
Final comment: I am occasionally asked why I carry this knife or that, and why I "obsess" about keeping them sharp. My standard answer is that "you just never know when you might have to cut off your own arm." Google Aron Ralston for insight into that line of thinking.