10 Monkeys cannot eat 5 Bananas
At some point during Marginal Changes analysis the marginal rate (slope) is starting to decrease. Each additional unit is not going to add as much as it did before.
Marginal Product … utility of adding 1 unit Average Product … average utility of all resources
At the cross section of those 2 functions is the sweet spot of resources to use. Everything less will have an opportunity cost and everything more will not be as efficient as it can be.
This is about efficiency and therefore about profitablity.