Hum,
Well simply, by looking at the size of the crater it left was an easy way to find out what it was made of…
But a lot more can be found out by looking at the spectral (light) from the dust.
So we now know its composition and make up…its very dusty, and not solid…which may help us out in the future….
The main reason for the mission, imho, is to avoid the fate that befell the dinosaurs…
So quite cheap compared with that.
Landing on a comet and taking samples from lots of location would be a better option, and there is one on its way to do exactly that.
Europe's
Rosetta space mission will land a probe on to the comet Churyumov-Gerasimenko in 9 years time…
Mark it in your diary…