Stockbrokers and banks know that a client who has a single account is more likely to switch providers than a client who has multiple accounts. The more products and services a customer buys from you, the higher their switching costs and the more difficult it is for them to change suppliers. Banks, for instance, strive to sell you a variety of services—checking, savings, credit cards, mutual funds, computer access, and so on—partly because the more services you buy, the greater the hassle for you to change banks.
The same sales principle holds true for all the salespeople I’ve worked with. They need to diversify their risk by increasing the customer’s attachment to their company. This principle works as long as those additional products and services increase benefit to the customer. As they continue to add value by implementing multiple solutions, they become a more valuable supplier for their customers—either because what they sold them second is integrated into what they sold them first, or because the knowledge they continue to gain about their needs and opportunities helps them add more value.