Farmer

Farmer is a person who cultivate the fields or look after the animals and make sure the plants or animals are well taken care of and growing well. Not only the farmer feed the plants with fertilizers and water, he also feed the animals with proper nutrition and ensure they stay healthy.

A farmer-type salesman basically does the same thing. He looks after a group of customers and make they get regular care and feeding. He also have to make sure the customers are growing and growing well. If any customer is "sick" it is his job to get them well again. "Sick" may be poor business performance due to their own business environment or buying from competition. The salesperson's(a more neutral word than salesman) job is to troubleshoot and help the customer to get well and buy more from the company.

If all is well, then the salesperson job is to maintain the well being of all customers and spend time to figure out how to increase the relationship further so that the customer will buy more.

It is so simple.