Because the guy you want wool clothes from wants beef instead of the lentils you chose to grow, and the person who wants your lentils only has chickens, and the guy who wants chickens has cabbage, and the guy who wants cabbage builds houses, and the guy who wants a house has pork.

Money exists to make transactions easier. Barter sucks as a primary trade mechanism unless everyone already makes almost all of what they need themselves and the barter is just on the side to spice things up a bit.