...A means to pump air...
Times two.
One of the last things I thought about for my on-the-road kit.
Yeah, you can carry a spare, and a jack, and tire chocks - all of which I do.
But.
Most of the time, you've picked up a nail and have a slow leak, which you only notice when the tire is too low to drive on. You could hassle out putting on the spare, but, well, hassle.
If you could just make it home, or to a more convenient spot to do the wheel change, or to a tire shop that could do a proper repair, life would be good.
If you ride a bike, you know about these:

I like the Topeak Morph, but there are a bunch like it. This takes up no space and has a few cute tricks. A real handle, A little foot peg. An inline gauge. An extendable hose that will actually reach the valve. No wires to hook up. No expendables to replace. Simple. Neat.
Yeah, you will be pumping a lot to pressure up a car tire, but how often will you need to do this?
Besides, cheap insurance. If you have it with you, you'll never need it.