A piezo ignitor will allow you to relight your water heater pilot light by simply pressing a button. This can make lighting the water heater in a dark basement much easier since you don’t need a long match or lighter.
The piezo ignitor can sometimes fail on a water heater which will prevent you from being able to properly re-light the pilot. If this happens it may be possible to light the pilot manually until you can replace the piezo. Depending on the type of water heater, the Piezo may have to be shipped out which can take up to a week.