The depth of field (DOF) is the area of the photo that is in focus. The things you can change to alter the depth of field are to alter the aperture, the focal length, or the distance from the subject.

I’m very fond of very tight depths of field (with portraits, often I have the eyes in focus, but the eyebrows are already out of focus), but at times, it’s good to use larger ones.

By having a larger depth of field, you improve the chances of the shot being in focus, you show the whole of a person (if it’s a portrait) in focus, and finally, and most relevant here, is you can show the person within a context or environment, as opposed to just being surrounded by smooth blurred colours.