This old formulation method still has uses in commercial situations
Imagine being a farmer in some Midwestern state of the U.S. in the previous century, and you had only three ingredients to feed your animals: corn, soybean meal, and a supplement. The supplement contained nothing more than some vitamins; trace- and macro-minerals, including salt; perhaps a medication; and one or two other additives of no importance for this discussion. To make life easier, the nutrition supplier said you could use the supplement at 5% in all of your final feeds. The rest was up to you because that nutrition supplier wanted you to have freedom in making your rations.