Long ago, zed cards were not affordable to create, and design tools were simple. Most new models didn't have the capital to print composite cards, so they started out with an uncomplicated 8x10, B&W print with a thick white outline. Vital stats were put on to the reverse side in plain text.
I can still remember those good old days, printing tons of copies of the same boring picture. So many times. These 8x10 photos were also somewhat expensive, and this impeded the model handing out them out. Models typically sent them to companies who were likely to offer them a gig, or to casting agents who were likely to use them. Models definitely missed out on some assignments due to the fact that they couldn't afford to pass out headshots to just anyone.
Later in her career, a model would grow to be more successful and pocket more moola. This would allow the model to order a one-color zed card produced by an offset printer. Only the most successful models in New York City could pay for full color. Offset printing requires a lot of money up front, but the cost became cheaper if a print run of hundreds or thousands of sed cards was done at once. Now, a model would have plenty of cards available - and the model could more readily afford to send a card to anyone who might be only somewhat interested in hiring the model. The comp cards were even cheap enough to use the postal service to send to casting agents around the world, extending a model's market.
The sed cards of yesteryear were a specific way due to the technology and costs involved with printing. This meant a single headshot on the front and a handful of photos, all a quarter of the available space, on the reverse of the card. A location was also reserved on the rear of the zed card to hold details for the model and a method of contact.
Printing techniques got in the way of the photos on the reverse of the sed card from touching in any way, and you weren't able to use any cool backdrops or layouts. All sed cards were therefore made on a white background, with thick white borders. These outlines also enabled the printing press to hold the zed card as it traveled through the printing process. They couldn't bleed to the edge, the way modern cards and layouts do. Despite the fact that printing technology has come so far, the zed cards we design in the present are still founded pretty solidly on this traditional design, which resulted from the realities of printing.