They hired marker comp artists. Artists who would sketch an art director’s idea in order that the concept could be visually presented to a client for approval. I did lots of marker comps for clients in the 80’s.
You never knew what someone would ask you to comp – fashion, food or mechanical contraptions. Remember, no computers, no Google. I had file cabinets full of reference, pages I tore from magazines, anatomy books so I could correctly illustrate a figure in perspective. And I had hundreds of magic markers – nearly every color produced.
I spent many all nighters sketching marker comps for clients and agencies. It was stressful work, laborious work but it was quick. I might get a call in the afternoon but had to deliver comps to the agency before 9 am the next morning. When computers and scanners and the internet became common place, my freelance marker work dried up as did all those hundreds of magic markers.