What are frets used for?

On a fretted instrument, the fret serves to shorten the string's vibrating length when a string is "stopped" at it.

The ancient Germanic tribes and the ancient Scandinavians used runes as an alphabet. Prior to that, they used runes for magic purposes, such as casting lots (divination) and spells of protection.

A model aeroplane is used for presentations and testing. Model planes are also used for fun. They are used model-plane clubs, aeroplane company headquarters and studios. (Bedroom, in my house)

That people wont work.

The only answer that comes to mind is "Airships". Hydrogen, the lightest element, was used in airships, where excess weight was undesirable. However, accidents like the Hindenburg disaster showed...