Vitamin K is pretty interesting, and is involved in a lot of functions. The body uses it for calcium/phosphorus partitioning (as well as other things). It keeps the calcium in your bones and out of your arteries, this prevents Osteoporosis and Atherosclerosis. For me personally, after using it, I noticed that my hair and skin were softer, and that my teeth stayed slippery and clean longer. Its absence from modern diets is a partial cause of many modern health problems.
Vitamin K1 is made in plants as part of photosynthesis. Animals eat the chlorophyll in plants, acquiring Vitamin K1 which gets converted to Vitamin K2. Since cows eat only corn nowadays, our meat, cheese, butter, and milk have little or no Vitamin K. You can get vitamin K1 by eating leafy greens, but you must eat a significant amount, and even then your body must convert it to K2 - which is not a very efficient process.
One thing to note: Blood thinners work by eliminating Vitamin K. If you take blood thinners, do no take this vitamin. It will make blood thinning medications ineffective.