I come from a sailing and climbing background: those two sports are all about knots. You need to know your knots really really well for both sports, but especially climbing as you are 100% reliant on the knots you tie. Beyond that, knots come in handy all-the-time ... it blows my mind how many people couldn't tie something securely to the roof of their car - for instance. In climbing, there are many knots that can perform the same function as a piece of hardware. A prusik, for instance can function like an ascender, it's not as efficient as a mechanical ascender, but in low-volume use or emergencies - they're almost as good ... a lot cheaper ... and much lighter. Often when I'm mindlessly watching tele, I'll practice knots. I think if I could only know one knot, it would be the bowline - very versatile, strong, and easy to remove.