The design of any given headset means that at a minimum you'll need a certain length of steerer for all the parts to be able to screw on properly.

The sum of (the distance from the crown race to the bottom headtube race) and (the distance from the top headtube race to the top of the lock nut) is called the stack height. It varies depending on how the headset looks and is designed.

If you have a fork that was trimmed to properly fit a very tall stack-height headset, and you want to switch to a low stack-height headset, then you need to use spacers to take up the extra space (or trim your fork). If your fork was trimmed for a low stack-height headset and you want to use a tall one, you are out of luck -- there just isn't enough space.

Here's a picture of a tall one:

Here's a short one: