Ever stop to think about what the properties of a particular focal length are - not the lens itself but simply what does it mean to shoot with a 100 mm. lens as opposed to, say, a 25 mm. lens?
Obviously the difference is one is telephoto (or at least long focal length) and the other wide angle. They have obvious implications about close focusing and depth of field etc. but beyond that the nature of lenses impacts the look of a photograph.
Lets say that you are photographing a 3/4 length portrait. If you photograph it with a wide angle lens, you have to stand closer than if you use a long lens. The depth of field ends up the same so that isn't the difference. Two things occur. Anything closer to the camera is portrayed larger, whether it be a hand or nose. Were you to stand back and use the telephoto, the relative difference in nearness of the hand and nose are smaller so less distorted.
This can be used in landscape work where there are two objects, one in foreground and one in middle ground and you want the foreground object to dominate. On the other hand, if you want it the other way round, then stand back and use a longer length lens.
Changes in the appearance of the image are strictly a function of position of the camera, not the focal length. The change in lens simply determines how well you fill the frame with your subject from a given position.
The other characteristic of the lens is how much of the background is included. For example if you are 4 feet away from the subject and your lens encompasses a width of 5 feet at this distance, at double the distance it will include twice as much but double the distance is only 4 feet behind the subject.
With a telephoto, you might be 20 feet away and encompass the subject 5 feet wide, but at 4 feet behind, the coverage is about 6 feet, not 10 like in the wide angle example.
this of course is simply the foreground issue turned round, but it does mean that when you want a simple background, you are more likely to find one with a telephoto than a wide angle which includes so many more things in the background that the chances that some of them interfere with the image is that much greater.
As an example, your sitter is in front of a weathered barn door. With the telephoto the background is the door, with the wide angle, the background is the barn, the telephone pole, part of the tractor and the dog's tail. Perhaps you wanted an envoronmental portrait, but maybe not.