Brix was originally a measure of sugar content in a liquid used in the juice, dairy and beverage companies. The refractometer is a quick way for the worker to get a brix reading without having to wait for a lab test. In the beverage industry the use it to check end of fermentation as it gives you the amount of unfermented sugars. Juice industry uses it to check proper concentrate level. Dairy plants would use to check % solids when evaporating milk or whey. Most companies make there own charts to use with more advanced lab equipment. Every liquid is different and requires a chart to compare to. A Miller Lite would have a different chart than say Bud Lite.
What use it has for food plotters I don't know, Do they corrolate the measurements to % protein, fat content, or just to the calories of the plant? All of those change quite a bit as a plant matures.