Focus, in the sense of focusing the lens, is one of the primary variables in a photograph under the control of the photographer. Assuming, that is, that the photographer doesn’t rely on autofocus, and understands how focus relates to depth of field and aperture.
Focus in life means concentrating, and screening out distractions. Sure, as a technical matter, you focus the camera (or it focuses itself). A good photo composition emplys focus in the broader, non-technical sense as well. This kind of focus should tell the viewer what is important about a composition.
Ansel Adams put the gist of composition well: “I think in terms of creating configurations out of chaos.” Your composition shows what your photo is concentrating, or focusing, on.
This focus might be a broad, distant view as in my first photo of the lily in the green vase (below).
In contrast, a photo might focus on an extreme close-up of a small part of the flower, like the image of nectar oozing aroung the stigma of the same lily (at the beginning of this story).
Using focus in this non-technical sense means deciding on a sense of scale, and more importantly, being clear about what story you want to tell.
[Nectar of Lily: Nikon D300, 200mm f/4 macro lens (300mm in 35mm terms), 36mm extension tube, Nikon 6T close-up filter, 3 seconds at f/40 and ISO 100.]