Unless you are a non-profit, the point of the business is making money. It is surprising how many different ways of making money there are out there, but if you are in business, it is extremely likely that you make the money the easiest and the most common way around: by selling goods.
There are many ways goods come to be, but from perspective of a single business goods can be either purchased, or manufactured. But even companies manufacturing the goods they sell have to somehow get hold of the raw materials used to produce their goods. The most common way to get the raw materials for your manufactured goods is to purchase them. So in the end of the day it comes down to the simple fact that the most common way of making money is by purchasing and selling goods.