Thunder & Lightning has (barely) one practical purpose: to measure your distance to a sound source. It works for thunder and lightning, fireworks, rocket launches, or other loud events visible from a (hopefully safe) distance away. Just press the Lightning button when you see the event, and Thunder when you hear it. Modern technology does the rest.
To perform this trivial task even more accurately, you can enter the temperature of your surroundings to more precisely calculate the speed of sound. You can also choose between U.S./Imperial and metric measurements depending on your personal unit preferences.
Should its intended purpose be insufficiently useful, this application can also be used as a reasonably precise (if redundant) stopwatch, a speed-of-sound calculator, a Fahrenheit/Celsius converter, or as an odometer if you happen to be traveling at precisely the speed of sound.
Note: Use of application while operating supersonic aircraft is not recommended or approved.