Adjustable coilovers vs spacers question

Ok so I know King and Icon coil over shocks are way better then a 2 inch spacer you put on top of a shock. But my question is why? Doesnt adjusting the King shock to add lift essentially do the same thing as a spacer? I always hear you can not crank the kings or icons too high or it preloads the springs too much and makes a rough ride. Isnt that what a spacer on top of the coil does as well?

Again, this is not about whats better. I know coilovers are better. Just trying to figure out how they work.

Thx