When a calf (or any baby animal or person) is born it does not have much (if any) of an immune system. Every virus out there can attack and kill the calf. To offset this lack in the immune system, mother nature provides colostrum from the mother which contains antibodies which fight off the viruses. So while the cow is pregnant her body is exposed to the various viruses in her environment. SHe then develops antibodies to those viruses. When she produces colostrum, it contains those antibodies. When the calf is born it begins taking in viruses. The cleaner the environment the fewer viruses it would take in. Regardless, it still will take in the viruses. When it nurses it gets the antibodies from the colostrum to fight the viruses. Now here's the unfortunate part. The calf can only take in the antibodies from the colostrum in the first 24 hours. After that its stomach won't allow the antibodies to enter into the bloodstream. Actually it is worse than 24 hours. As time passes, the ability to take in antibodies drops rapidly. Your best chances are in about the first 2 hours or less. Each passing hour dramatically lowers the ability to absorb the antibodies from the colostrum. That is why you will see the literature tell you to have the calf nurse in the first 1/2 hour. The earlier the calf nurses the better the ability to absorb the antibodies. After 24 hours you are pretty much done with the intake of the antibodies (probably closer to 12 hours but I think I have had calves that went over 12 hours and still made it). If the calf did not get any colostrum in that time it normally gets a virus that kills the calf since it cannot fight it off. Seems like all the antibiotic meds that humans have found or created still won't save the calf like colostrum does. In other words. Without the colostrum you usually don't have any chance (even with shots) of keeping the calf alive.
The colostrum also contains a lot of nutrients as well. The calf is born with a tank (stomach) that is just about on zero. You need to fill up the tank as soon as you can or it will go on zero and then you have a calf too weak to nurse. Colostrum is great as a fuel with all its nutrients and therefore energy. The cow usually produces the colostrum for about 2 to 3 days. This gives the calf a high energy/nutrient start to life.