They are called strikers and are used to light gas powered torches such as Oxy-Acetylene or propane. You can get a flame out of them as I have used them to light cigarettes at work when other means were not available. Usually sprayed some paint or other flammable substance in the cup and squeezed the striker and enough flame to light a cigarette.

As far as a one handed firestarter They could be rigged to work but I would think it easier to buy a sparklite or Blastmatch or carry a bic.

The replacement flints can work but are kind of small you may be able to buy some flint rods in that diameter online.

The triple flint striker is nice I use one at work and like it much better than the single flint version only because you have replaceable flints on the sriker and all you do is rotate it after loosening the screw to have a fresh flint.