The Highwayman's Hitch is a wonderful, special-purpose knot. It is useful when you need the boat tied up for a short time, and you need to undo it quickly from a distance.
I use it all the time putting a boat into the water by myself with the hoist, but it is also useful when you are doing this with a helper in low tide. In that case, you generally lower the boat with the bow line either secured to the ramp or held by someone on the ramp. Then you have to get the line down to the dock to secure it there.
How do you do that? The wrong answer is drop the line and run around a get it, as the boat should be under control at all times. If the tide isn't too low, you could tie it to the ramp low enough to reach up and untie it from the dock, but that't not always possible. The Highwayman's Hitch is a more elegant solution.
I won't explain how to tie it, except to point to the Animated Knots animation.
You tie the knot around something on the ramp and drop the working end down. You tie it "on the bight" which means you don't need the end of the line to tie it. I just drop the end of the line (the working end) down and tie the knot. When it's tied correctly, it will hold the line to the boat securely, but it will come undone if you pull hard on the working end. So it doesn't matter where you tie it, as long as you can reach the working end from the dock.
You do need to be careful tying the knot. If you do it wrong, the boat pulling on it will undo the knot, and you will have an interesting situation to deal with. So always test it by pulling against it on the line going to the boat (a good idea whenever you tie the boat to the dock with whatever knot).
In our recent "Between the Docks" workshop, Brenden Kallaby pointed out another great use for the knot in departing single-handed. This can be a challenge on a Quest. Typically, you untie the boat and cast off as you step on the bow. You have to get around the mast, through the upper and lower shrouds, and get control of the boat. I teach a reliable way to do this, but it requires stepping between the upper and lower shrouds, which requires some practice and good weight placement.
I've seen people run the bow line through the ring at the dock (or around the bar at the middle dock) and bring it back with them to the cockpit. They then release it, effectively casting off from inside the cockpit. This method has a significant problem in that you're dragging almost the entire bow line through the ring or around the bar, and it could snag on something.
Here's where the Highwayman's Hitch comes in. Just use it to tie the boat off to the ring or the bar, and bring the working end back to the cockpit with you. You cast off by yanking hard on it, and away you go in full control of the boat. It's very unlikely to snag, as there's very little line between the dock and the boat, and the line isn't wrapped around anything.
The Highwayman's Hitch is one of a class of knots with similar characteristics, including the Tumble Hitch, the Siberian Hitch, the Mooring Hitch, and the Cavalry Hitch. Of these, I would only use the Tumble Hitch as an alternative in single-handed departure, as the others either require using the end of the line to tie them or when released have the line wrapped around the ring or the bar.
Unfortunately, there's no equally easy solution for single-handed docking with limited space (when you can't turn sideways at the end). You just have to learn to go forward and step off, which is what we do in our Single Handing Workshops.