Here's a good explanation for the reason of grinding coffee just before brewing, by Counter Culture Coffee.
http://www.facebook.com/notes/counter-culture-coffee/the-craft-of-coffee-grinding/454443492933Particularly liked this:
"You see, those aromatic oils are volatile – they escape easily from ground coffee and fill the air with one of the most beautiful scents on the planet. But we want those aromatics to go into the cup to create coffee deliciousness. The longer coffee grounds sit around, the more oils escape into the atmosphere, and the fewer are available to be extracted into your cup; it’s as simple as that. Grinding in your kitchen at home has a double benefit – not only do you preserve most of the oils for brewing by keeping the interval between grinding and brewing short, but what aromas do escape fill up your kitchen,
making your family a little happier and more cooperative."
