What is the main role of vitamin k?

Most vitamins have more than one function and I think this is true of vitamin K. The one I always think about though is blood clotting. So it helps forms a scab when you've been wounded. It also has a negative effect in that blood clots in arteries can be fatal so a lot of medicine used to thin the blood (after a heart attack, say) also stops blood clotting for good purposes (like wound healing).