Salt, along with other minerals, are necessary to sustain life in cows. Much like humans they are the essential building blocks for a living being to perform at best.
The interesting thing about salt is that cattle have a natural “urge” for it. This means that unlike the other minerals they will actually seek it out. If they can not find it then they will try to ingest it from other sources, such as tree bark, dirt, and even urine.
As I mentioned earlier salt is very important for the overall function of a cow. Along with aiding with milk production it will assist with other functions. These functions are things like reproduction, blood flow and even sweating, which I think that one should be obvious.