Why the next glass is worth less
Economists call the extra benefit from one more unit of something marginal utility. When you are very thirsty, the first glass of water can relieve a strong need. As you drink, that need shrinks, so another glass has less to fix. The water itself has not changed; your situation has.
This is one reason people do not value goods only by their total usefulness. Water is essential, but when thirst is already satisfied, an additional glass may have little immediate value compared with something else you could use your time or money for.
The same logic shapes choices
Diminishing marginal utility helps explain why consumers spread spending across different things. If each additional serving of one food brings less enjoyment, a person may prefer to spend the next dollar on a different food instead. Businesses and economists use this idea to think about demand: people are often willing to pay more for the first units that meet a need than for extra units they already have plenty of.
The principle is about the next unit, not whether the good is useful overall. A glass of water can be crucial when someone is thirsty and nearly irrelevant a moment later—which is why value depends on what the person needs now.
