A salesman's value is not always quantifiable. Helping someone and making him or her feel valued and respected is not always something that can be placed on a spreadsheet and graphed. Dealing with people so that they get what they want does not mean zero effort was put into the transition. Sales is a matter of trust and trust builds with time as one proves to be competent.
There isn't always the possibility to add to a sale. A salesman needs to follow his intuition. Some people know what they want and don't want advise on what to purchase. Helping these people quickly through the process is what they are looking for and that in itself is the value brought to the sale.
The purchase has to happen somewhere. So whether this salesman takes the order or if it is someone else, it has to be done. Nothing happens until there is a sale. Seeing the salesman's value takes a bit of thought and understanding as it may not be readily apparent on the surface.