The idea that Adam Smith had expressed as "natural price" has become the centerpiece of modern economics: the theory of supply and demand. Smith had claimed that the forces of free competition and profit-seeking would move the market price toward one specific, "Natural" price. We now think of that price as the "equilibrium" price, and understand that the equilibrium price is determined by the interaction of supply and demand. To better understand how this happens, we need to learn more about the nature of demand -- what is it that makes people willing buy, at a price? While this discussion has a long history (and to some extent philosophers had worked on it before economics came into existence), the discussion got rather hot in the 1880's, and a new approach that came to be known as "the theory of demand" became a key part of microeconomics.
In a way, the surprise is that anything at all can be said about this, in general. The obvious guess is that people have a wide variety of motives, and that different things motivate different people to buy different things, so that nothing can be said in general. All that these motivations have in common is that the item bought gives satisfaction -- or, as we will say, "benefits."