Fire can save your life many ways. Light by night and smoke by day to signal rescuers, melting ice and disinfecting water, detoxifying, cooking and preserving food, preventing frostbite and hypothermia, repelling animals (including flying insects) stopping bleeding and disinfecting wounds by cauterization, are some of them.
Ferrocerium (FeCe) spark producing rods are smaller than a box of matches, provide hot sparks, hundreds of lights, and work wet.
Large, more expensive FeCe rods are available with or without built-in strikers. But such rods without a support of any kind will break when they are worn too thin and so cannot be used up as nearly completely as those bonded to a supporting structure, such as a magnesium (Mg) block, for example. So, besides being more expensive and wasteful, they also have unnecessary weight.
Those with built-in strikers usually use an unnecessary amount of the rod per strike and do not direct a lot of the sparks most efficiently for lighting the tinder. Built-in strikers are best for those who do not realize how easy it is to use a separate striker and how efficient it can be in lighting a fire.
With a separate striker you can hold the striker stationary in line with the tinder, and strike with the rod, so that nearly all the sparks hit the tinder. The rods supported in a magnesium block are smaller, and less expensive and the magnesium can be scraped to make fine shavings that produce intense heat (to 5,610 °F) and brilliant white light (including UV) when ignited. Besides allowing the ignition of very hard to ignite materials, these shavings could be used as a flare. The magnesium can also be used to produce hydrogen for loftingballoons if attacked by acids or maybe alkali.
These FeCe-Mg firestarting blocks are often sold for around 2.00 USD, at Harbor Freight, for instance. Also, magnesium is a very light metal, having a density of 1.738 sp.gr. These firestarters come with what looks like a short section of a hacksaw blade. But any really hard material of the right shape will do: tool steel, spring steel, flint, WC, etc. I would rather carry extra FeCe-Mg blocks than one of the large rod systems. This would also usually be cheaper.
The drawing below left shows the ground surfaces in pink, for a good shape for the striker.
The drawing on the right shows the FeCe rod stationary and the tinder (as from a tinder tube) being held and moving with the striker.
