By maintaining a relationship with your customers, you increase their awareness of you, and consequently retain better sales. Re-selling and cross selling are where the real profits are made in any business. An existing customer is already pre-disposed to buy from you again. Microsoft and other large successful companies do this all the time.
Humans are creatures of habit.
This is especially true if the item were on sale for half price, for example. You get more customers into the habit of coming back to your site so they are more likely to bookmark your site. This is where the real money is made. This is how you can capitalize on the lifetime value of a customer. To maximize the potential of this concept, you need an automated follow up system that does all the work for you.