"Comets are particularly important because they, or rather others more or less like them, may have originally provided most of the water and organic chemicals to Earth 4 billion years ago.They provide a key record of what conditions were like 4 to 4.5 billion years ago. Comets also occasionally hit Earth in modern times. Fortunately, not very often and specifically none in our lifetimes, although a few people still think that the airblast above Tunguska in 1908 might have been a comet."