All sound is vibration. Of common objects, tensioned springs are among those most easily stimulated to vibrate (the definition of a spring, according to wikipedia, is an elastic object used to store mechanical energy). As with steel generally, springs tend to incorporate the the sound qualities of the object the are excited by into their own.
Musically speaking, they are very long strings whose length and sensitivity allow them to resonate sympathetically (the reason they are often used in reverb units) or generate their own sounds. They generate a fundamental pitch (below the threshold of hearing when they are long) and, owing to their excitability, numerous overtones.
Some springed instruments of Anarchestra:
Sprawl 05-6 Santa Fe
Sacre Bleu 05 Santa Fe
‘Sprong 06 Santa Fe
Soren 10 Tucson
Springbok 10 Tucson
Sprix 13 Tucson
Sprymbal 14 Tucson
A few other instruments have springs incorporated into them to add resonance/sustain. Many have springs as a mechanical component.
In addition they are sometimes useful as bows
Spring Bows 02-06 Chilmark, Santa Fe