Vitamin K is actually two different vitamins doing different jobs. K1 (phylloquinone) is the one in leafy greens, and your body uses it mainly for blood clotting. K2 (menaquinone) is the rarer one, found in fermented foods and some animal products, and it does something completely different.
K2 activates a protein called osteocalcin, which binds calcium and pulls it into your bones and teeth. It also activates matrix Gla protein, which removes calcium from your arteries. Without K2, calcium that gets absorbed (especially when you're supplementing Vitamin D3) doesn't end up where it should. It can drift into the soft tissues, including the lining of arteries.
This is the calcium paradox: high calcium intake without enough K2 has been linked to harder arteries even while bone density doesn't improve as much as it could. K2 is the traffic warden directing where the calcium goes.
Most multivitamins leave K2 out entirely, partly because awareness of it is recent (the research really started in the late 1990s) and partly because the MK-7 form is more expensive than D3 or C. We include it because if you're taking D3 anyway, K2 is the partner it needs to work properly.