This is a good question. There are several conditions where personal selling becomes central to the cultural of a company. This would be a true statement in any company that is in sales. Here are a few examples.
First, the most obvious example of the importance of personal selling can perhaps be seen in the traveling salesperson. Two companies come to mind immediately. Avon is a billion dollar company that prides itself on selling door to door or by inviting people over to have demonstration of their products. Cutco knifes are very similar. The skill of the person who is selling is central.
Second, any company that works off of commission will have a strong emphasis on selling as well. For example, financial planners often work off a percentage of what people invest. For these people, personal selling would be central. Many retail places work the same way.