Nice for filling up air mattresses, bike tires, blowing dust out of your computer, etc etc. This is a useful thing to have.
If you ever get your car stuck in soft sand, snow, or mud, there's an easy way to get out. Just let air out of your tires til they're at about 10 or 15 psi. They'll have a nice big flat spot on the bottom, won't sink in nearly as much, and you'll drive right out. The military think this is such a useful trick that many military vehicles are equipped with gadgets that can drain and fill the tires from inside the cab. On Cape Cod there are still some beaches you're allowed to drive on. At the entrance to one such beach is a shed with an air compressor and a sign telling you why to flatten your tires a bit.
Without such a convenience but with a couple 100psi tanks of air in your trunk, you can refill your tires quickly after you're done driving on the beach.

Step 1: The Plumbing

Get an empty freon can from behind a garage or air conditioning contractor.
They will have pumped out all but a few molecules of nasty gas using trade school magic.
Make sure it's a really empty one by using your own brain.
Otherwise teenage nerds will heavily criticize these instructions.

Get your fittings and put them all together. You'll probably have your own scheme in mind which will be different from mine.
Wrap the threaded parts with teflon tape before assembling to seal the threads. If you don't have teflon tape use a strip from a plastic shopping bag.
Attach your hose to the tank with a hose clamp.

Step 2: Defeating the Check Valve

This can had a check valve. When I tried to fill it no air went into the tank, even with the valve fully open.
To take the valve apart I had to grind through the lip around the edge with a dremel tool. Then I unscrewed the knob til it came off. Inside the valve body was a ball bearing and spring. that was the check valve. I pulled them out and reassembled the valve. Now I could fill the tank.
Two views of the top of the valve with the ground-off lip shown.

Step 3: The Attachments

The hose coming in from the right comes from my air compressor. To fill my tank I use the double-ended nipple dingus in the middle. If you're filling your can at a gas station cut the stem off a bike innertube and either tee it into the airhose to your tank or hose-clamp it to a quickrelease nipple. The other two dingii in this picture are a blowgun and a tire filling attachment. I like the quick release stuff because I already have it and it makes experimenting quick.