Put it this way -- when a calf is born it has no immune system, no digestive system and nil friendly bacteria in its gut. Initially all the calf  feeds on is the colostrum and then, after 3 days, ordinary milk from the mother cow. The colostrum is very important because the mother cow passes all her immunity into the calf via the antibodies in the colostrum and this kick starts the calf's immune system. The colostrum also kick starts the calfs digestion and flora as well. Exactly the same thing happens when a human supplements bovine colostrum from the mother cow. 

And if the calf gets no colostrum from the mother cow within 24 hrs then it will die. Ask any dairy farmer or vet.