Salt, along with the other minerals, is necessary to sustain life in cattle. Much like humans they are essential building blocks for living beings to perform at their 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 cannot find it they will try to ingest it from other sources, such as tree bark, dirt, and even urine.
As mentioned earlier salt is very important to 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 should be obvious).