Keep in mind as well that I'm speaking within the parameters of a capitalist system.
Objects/items/etc. are not inherently useful, because useful is an egomorphic, Human-oriented term. Say we have a hamburger sitting in front of us, on a bun, with lettuce, tomato, the whole deal. We understand that object on the basis that it is potential food/sustenance for us. It's useful to us because we need food.
This use-value is not a character trait of the object itself. It's a trait we, as humans in need of it, attribute to it. It alone (w/o humans) does not have the use-value of food.
Now, let's not get so anti-humanist. Say there are still people and the burger is in demand. People are hungry, they need food. A burger possesses no use-value for its owner who is willing to sell it; otherwise s/he would not sell it. It only possesses use-value for those who are willing to buy it. Therefore, an exchange must take place, and this exchange must take place before any use-value of the burger is realized. The buyer must put something up in exchange for the burger, something that is accepted as equal in value to the burger: money. Does money possess the use-value of a burger? No, not at all. Something that is utterly different from an item's use-value is being realized during this exchange, and Marx qualifies it as the exchange-value.
This exchange value functions as a symbol, and commodities must be dealt with as this value before they can be consumed as a use-value. When you buy a burger, you don't say: "That burger is the equivalent of my hunger's needs" (or, you might, but that won't get you anywhere). You have to say: "That burger is the equivalent of $1.99" (or whatever). You equate $1.99 with your hunger, but this is an arbitrary, merely symbolic equation. You have to recognize the product as an exchange-value before you can realize its use-value.
