To determine whether an additional unit of a good or service should be bought, made, or used, you have to find out if the marginal costs are greater or less than the marginal benefits. Each decision has things that you gain and things that you give up. If you are looking at adding one more unit, you want the things that you gain to be more than the things that you give up. In other words, you want your marginal benefits to be greater than your marginal costs. Then, you are better off.