This is a great question that I often discuss with my own students. The salesman's role has changed greatly over time, and, sadly for him, during Willy's career.
When Willy entered sales fresh from school, developing relationships and being personally liked by the customers was important. Traveling to different territories and creating this personal client base was Willy's job, and his strength. He understood how to use personality and friendship to create sales. His ultimate relationship was with his boss, Harold Wagner.
As time progressed, though, the sales field became less about personal relationships and more about the bottom line - money. Willy's client friends began to retire or die, and he was no longer known at the companies he sold to. As a result, his sales dropped because the personal relationships had been replaced by faster, more economical means.
This idea is symbolized by Howard Wagner's wire recorder. The real person can be and is replaced by a machine. Howard, the owners son, is unlike his father and not willing to pamper Willy and his resistance to change with the times.