Shallow depth of field is often used to separate a subject from its background, but there’s more to it than that...
The standard use of depth of field is when a specific area needs to be sharp, but no more, as in a head-and-shoulders portrait with an out-of-focus background. In this case a medium telephoto lens was an obvious choice, both for flattering proportions and for the relatively shallow depth of field at this distance, which separates the head from the background. The important zone in a portrait is from the tip of the nose to the ears, typically a little less than 20cm. At this distance, f/1.4 gave the right depth of field when focused on the eyes.
When you’re photographing faces, the area from the tip of the nose to the ears should usually be kept sharp