Isolating the subject can assist in moving the viewers attention from the background to the main subject. IIn the case of the image below an aperture setting of f/4.0 and a shutter speed of 1/250 of a second was used. This low aperture with a 105mm telephoto lens helps to throw the background out of focus. The lower the aperture used the more the background will be out of focus. Long telephoto lenses will always tend to reeduce the depth of field. In the image below even the outer petals of the flower are starting to go out of focus due to the narrow depth of field.
In addition to the depth of field a dark background can also assist in isolating the subject by allowing it to stand out from the background.