Here is a quick post for you, in which I hope that i can show why you would want to take images at different focal lengths.
I have included a few shots to display how your subjects face can even look distorted at a small focal lengths.
Also look at how different the background can look at the different lengths.
All the following images were shot, where the model was the constant, in the same place.
The Aperture and shutter speed were constant in each image also.
I was the variable, moving back each time to try and take the same shot but with a longer lens.
As you can see, the face looks odd and distorted.
Starting to look better, Keep in mind what the background is doing.
This is a much better and singles your subject out. Compare this to the image above looking at the background. I used the same aperture settings in each image. This image's background is much more compressed and the subject looks fantastic.
You have to remember one thing though,
the longer the lens - the further away you are to your subject.....
or the following will happen....
