Your are NOT changing the focal length
It is just a guide of an equivalent lens
If you are as old as dirt and used to thinking focal length in terms of a full frame Nikon F 35mm film camera and have just bought a D7000 you want to know the equivalent field of view
if you put a 50mm lense that you used on your Nikon F, on your new D7000, it will have the Equivalent field of view as an 80mm lens on the F
but it is still a 50mm lens and laws of physics have not changed
And I know that. But why do we want to know the lenses' focal lengths? I want to know what my image is going to be like, not what the gear is. The focal length doesn't change. The fstop doesn't change. But the depth of field and field of view changes, and that's all we want to know. As long as I want to know the depth of field and field of view, I'll have to multiply the crop factor to the focal length and fstop to get a meaningful and fair comparison. Equivalent focal length and equivalent fstop shows a great way to know what kind of depth of field you're getting and what kind of field of view you're getting. Unless, all your life you've been shooting only with one format. It doesn't matter when you are not changing format, but it matters once you start comparing different cameras+lens combinations with different formats.
And that's the reason I said the word 'equivalent'. I have not once said the lens changed.