I started learning knots by having a sailboat where they are undeniably useful. However, after learning a handful, they end up being useful in all areas of life

I started learning knots by having a sailboat where they are undeniably useful. However, after learning a handful, they end up being useful in all areas of life wherever one needs to hold something to something else (in a car, on a bike, in the yard when it's windy, endless possibilities). To echo others, if you have interest to learn just one thing, learn the bowline. You can get through life knowing just that.