When a home is built, there are many different things taken into consideration. One of them is how water-tight the house is. Keeping water out is essential to prevent damage. There are many issues caused by water intrusion, such as wood-rot, pest infestations, and most commonly, mold. It is unfortunate to have to deal with these, so this is why vapor barriers play such a major part of your defense against water.
Vapor barriers can be made in the form of plastic sheets of different thicknesses, paper, or even as a paint. They are meant to prevent water intrusion, but they will only function if they are covering the area completely.
That means that if you have a patch of paint missing in an area, or if you have a rip in your water barrier, you will most likely still have water intrusion. This is why it is so important the job is done well.