From BBC news:

Comets - giant "dirty snowballs", as some have called them - are believed to contain materials that have remained largely unchanged since the formation of the Solar System 4.6 billion years ago.

Scientists hope that by getting "under the skin" of Comet Tempel 1, they can gain new information on the Solar System's original composition and perhaps even how life emerged in our corner of the Universe.
You're right... NASA is positively fucking awesome.