That's an accepted fact. The unaccepted fact is that most businesses don't use all the opportunities available that will bring them additional business.
Second sales are add-on sales, repeat sales and sale by referral.
They are important because they are much less expensive to get than first sales. There are many ways to segregate customers. However, these same customers have other buying situations in that there may be things to go with what is in the planning stage. Everyone has things they would like to do should the opportunity to present itself. When you concentrate only on the current buying situation and do not try to ascertain if there are products or services to go with their other buying situations, you close the door on possible second sales.