Telephoto lenses are best for magnifying subjects that are far away. Often they are used when you cannot be physically close to the subject you want to take a picture of. For example, with wildlife, it’s prohibitive in many cases to be physically close. Using a telephoto lens has the effect of bringing wildlife visually closer without actually having to be there physically.  Another reason for using telephoto lenses is the positive effect they can have on blurring the background and helping to isolate the subject. When using a wide-angle lens, it’s harder to isolate the subject because with the increased field of view comes an increased apparent focus of the entire scene. But with a telephoto lens, the photographer can narrow the field of view, and with a phenomenon called telephoto compression can, in effect, make the background appear blurry and help isolate the subject.