Why do they give extra vitamin K to babies?

Prevent bleeding. An injection of vitamin k is given to newborns shortly after birth to prevent "'vitamin k deficiency bleeding". This serious bleeding condition would occur in 0.3-1.7/100 babies without the vitamin k shot. Newborns do not have much vitamin k when they are born because it does not pass easily through the placenta and newborn livers are not able to store it well.
Prevent stroke. Babies are K deficient at birth and take about 8-10 days before their level comes up naturally to allow proper clotting. The manipulation of the baby through the birth canal or extraction via c-section can predispose the brain to reflect birth trauma as a brain bleed. No extra K= more babies with brain damage.Simple solution to a rare but significant problem.