These commercial vehicles (the Panel Truck and Canopy Express) were designed by GM for local delivery purposes. They usually came with a seat for one delivery driver. A second passenger was not needed so the company that bought the new vehicle saved the cost of a passenger seat. Yes, the passenger seat on the panel truck was a factory option.
On the Panel Truck, this gave even more room for carrying longer merchandise such as lumber and all was carried out with bad weather.
The Canopy Express had a full wood panel behind the driver to keep him out of the cold (if it had an accessory heater). Thus, a little less hauling capacity unless smaller items were placed through the open door where a seat might have been.
