It was the end of the mission for the orbiting space craft, so they figured they'd end the mission on a 'bang' (crash the craft into the Moon) and see what other interesting things they can discover about the lunar surface.
Lots more at spaceweather.com:
"The impacts are designed to excavate frozen water from the cold and shadowy floor of crater Cabeus. Moon water is valuable stuff. It costs about $30,000 to rocket a liter of water from Earth to the Moon. If NASA could find water already on the Moon, it would save a lot of money for future thirsty colonists. H2O also can be split into O2 for breathing and H2 for rocket fuel.
Evidence of water will be sought in the plumes of debris that billow out of Cabeus. The Hubble Space Telescope, NASA's Lunar Reconnaissance Orbiter, and several great telescopes on Earth will monitor the plumes for spectral signs of water (H2O) or water fragments (OH). Some results could be available only hours after the impacts, so stay tuned."
http://www.spaceweather.com/archive.php?view=1&day=09&month=10&year=2009
The actual impact was like Geraldo opening Al Capone’s safe,
a big dud
I find this disturbing, as if the moon has been violated.
I saw a few programs on History and Science Channels that showed enormous deposits of titanium and other elements. The idea stated on the programs is that our mission to Mars and other planets starts with lunar colonies. The scientists were sure that the cost and ease of space travel would be reduced once we figure out how to survive the moon.
I worry that a “tragedy” of space debris killing astronauts would end the program before it begins in earnest.