If you know that 80% of your business comes from 20% of your customers, you will be better able to work to keep that 20% of your customer base very, very happy.
If you are a retailer and you know that for each line you carry, 20% of the assortment will produce 80% of the sales, you are in a better position to determine exactly which styles will generate the most business and profit. (Remember, this means that 80% of the assortment will only produce 20% of the sales… so we begin to see something about assortment planning and how much of each style to stock in store… or in the case of a manufacturer in the warehouse.)
This is the Pareto rule… read more at Wikipedia http://en.wikipedia.org/wiki/Pareto_principle
This rule is not really exact but it is a great way to get an understanding of where you make your money in a business.