Parity was first introduced during the period of the New Deal where the U.S. Federal Government guaranteed the purchase of storable crops like rice, corn, wheat, and oats at or very near parity prices which had the effect of creating a minimum wage or living wage for farmers.
Today, farmers are only paid a fraction of parity for their crops. Going back to the year 2014, the National Agricultural Statistics Service reported that, at best, farmers only received 46% of parity for certain crops. Many crops were much lower than this amount. As you can easily see, farmers are only being paid half (at best) of what they need to earn a living. As a result, a system of subsidies was introduced which forces farmers to live on the government dole but still barely living at all. It should also be noted that, because of the structure of most Big Ag operations, major industrial multi-national food corporations suck up most of the subsidies. In addition, this form of subsidization causes consumers to pay twice for their food; once at the marketplace and again at tax time.
Parity would, of course, ensure that farmers are able to make a living wage and that the most important aspect of a national economy and population is safeguarded but it would also go great lengths toward breaking up multi-national operations, reducing farm size, returning farms back to real farmers and families, and encouraging the production of clean, natural, non-GMO food if implemented properly. It will also move toward diversifying American agriculture which is trending more and more toward monoculture, quite a dangerous direction for the country and for the rest of the world.