Zooming by walking
If you have brought a prime lens that is not long enough, you often hear the argument that you should zoom with your feet instead, i.e. walk closer to the subject to get the effect that you want. But does it actually work zooming with your feet? Both yes and no.
When you walk closer to your subject, you can make the subject take up more space in the frame as if you had zoomed in. So in that regard, you can zoom with your feet.
However, when you zoom in, two things happen: One is that the subject takes up relatively more space in your frame, but at the same time the angle of view is reduced as you zoom in. As you zoom in, you more and more become a like a horse with blinders: you can see less and less to the sides. This effect you cannot recreate when you zoom with your feet. A prime lens has a fixed angle of view.
Let me illustrate.
The first image below is shot at 70mm:
In the next image here, I have zoomed in at 200mm, but not moved an inch. All the change you see between the image above and the image below is due to zooming from 70mm to 200mm. I promise you, I did not move an inch, nor a centimeter. Notice how the field of view is significantly reduced, i.e. you cannot see as wide in the image below as you can above.
So, finally, I zoomed back out to 70mm and walked closer to the subject. I was here exercising the advice to zoom with my feet. I am not to praise myself, but if you compare the green leaves above and below, I did a fair job of getting the subject to (roughly) take up the same size in the frame. So I zoomed with my feet.
But notice how much of the woods you can see to the left and the right here in the third image, and compare that with the image in the middle above. When you walk close to the subject maintaining the focal length, you also maintain the angle of view, and even though the subject takes up more space in the frame, you get a lot of the background as well.
So zooming with a zoom lens is smart if you really want to focus on your subject and not have too many distracting elements, whereas zooming with your feet works best if you also want to include the surroundings or the scene the subject is located in.