It was most likely a stylistic choice and an attempt to capture a moment that looks more natural than someone staring into the camera saying "cheese." In terms of how natural it is, candid photography, where the subject is unaware of the camera, is the most natural. After that would be the subject being aware, but not looking straight at the camera while making a pose or giving a fake smile. Passport photo would be the bottom of the "natural list."

If you look at painted portraits, many of them have the subject in a semi-profile position. They are rarely front-facing, looking straight and smiling. They are usually in semi-profile (not completely), sometimes looking at the viewer sometimes looking straight (straight were their head is facing) and never smiling.

It is most likely a stylistic choice, brought over from portrait paintings. This has fallen somewhat out of favor for the mass public, with the average person having almost zero shots like that (but tons of selfies) but this is still done in professional and artistic photography. Having that said, some of the most popular, most iconic photos till today have the subject facing away from the camera (eyes may or may not be turned toward the camera), and rarely smiling, as is common with popular common photography.

Photography moved from being done by professionals and in studios or events with appointments to anywhere/everywhere/anytime/by anyone. So the more common types of photography (subject willing photography, not street, nature, etc) is posed, looking straight and smiling (wedding, graduation, etc) or candid, completely unaware or doing something while not looking at camera (cutting cake, playing sports, etc). You very rarely here someone say "pose but don't look at me."