I'm thinking beyond Cadillac/GM themselves. Perhaps the company that was contracted to haul these beauties wanted to use an image like this to show potential customers that they were hauling world class cars.
Convoy, back then, were definitely known as world-class. My late friend Dale drove for them, a LOOOONG time ago.
St. Louis huh? Wonder if they knew what they had? With their weather the poor thing has probably been a Budweiser can 4 or 5 times over.