What I was trying to get at was the questions people have been putting up on here all apply to standard road riding. What they FEEL when they are riding.
Whilst technically correct, you've missed a couple of points. You've mentioned them elsewhere but haven't "joined the dots".
If a racer wants to change his ride height, he'll fit a spacer. The actual ride height changed by adjusting the preload is fairly minimal.
Adjusting the preload means tensioning the spring earlier or later in the shock/fork stroke. I agree with your descriptions below, but what you're missing is the effect that dynamic weight transfer has on the suspension. Again, unless the spring is a progressive one, it won't get stiffer through its stroke, but the amount of weight and the way it changes as its applied to the shock affects how it reacts.
Imagine the preload set on minimum. Now get a body bouncing up and down on the seat. The ability of the shock to soak up the bounces is diminished because there's more free play at the top of the stroke. This means the effective force of the bounces gets bigger and bigger (a bit like jumping on a trampoline) until the shock bottoms out.
Now increase the preload and do the same. There's less likelihood of the shock bottoming out because its able to dampen out the bounces earlier in the stroke, preventing the body from acting as an increasing force bounce on bounce.
All of the above is assuming there is some compression/rebound damping in play.
That's the primary purpose of road bikes preload. Typicaly preload is increased when a pillion is added, because there's that extra force bouncing on the seat.
Correct preload adjustment also reduces the rocking back and forth you get as you open and close the throttle whilst riding along - keeping things stable.
I just want to add - Too much preload can make the tyre lose grip and induce a hi-side, so for anyone reading this who doesn't know - be careful if you decide to modify yours.