Find a scene that has depth. From a fixed position, take a sequence of five or six shots at different focal lengths without changing your viewpoint. (You might like to use the specific focal lengths indicated on the lens barrel.) As you page through the shots on the preview screen it almost feels as though you’re moving through the scene. So the ability to change focal lengths has an obvious use: rather than physically moving towards or away from your subject, the lens can do it for you. The other immediate difference between the shots is the ‘angle of view’, which also depends on the sensor size of your camera. Use the sequence to try to get a feeling for how the angle of view corresponds to the different focal lengths for your particular camera and lens combination. Which shot in the sequence feels closest to the angle of view of your normal vision?
Does zooming in from a fixed viewpoint change the appearance of things? If you enlarge and compare individual elements within the first and last shots, you can see that their ‘perspective geometry’ is exactly the same. To change the way things actually look, a change in focal length needs to be combined with a change in viewpoint.
A simple scene with depth to illustrate the point about the effects of zooming from a fixed position. Using a camera with a 1.5 crop factor I took a set of 6 images using broadly the same aperture and the images are straight out of he camera in RAW (converted to JPEGs).