To quote Wikipedia:

Asbestos is an excellent thermal and electrical insulator, and is highly fire resistant, so for much of the 20th century, it was very commonly used around the world as a building material.

It was later discovered to cause lung cancer, mesothelioma and also because of its fibrous structure; it breaks into lots of tiny little microscopic needles when agitated. Those little shards get inhaled and poke tiny holes in your lungs which causes Asbestosis (kind of like Emphysema for smokers).