Why import classes?

This might be a stupid question… but why do you have to import classes?
For instance:

import flash.display.StageScaleMode;
stage.scaleMode = StageScaleMode.NO_SCALE;

But if you leave out “import flash.display.StageScaleMode;” it still works.
I can understand it wouldn’t work with custom classes.

Anyone any idea?
Thanks & Cheers, Wim