. Another thing you point out is the length of the springs after increasing preload will be the same. This is false.... if say the bottom of your preload ring (where the top of the spring pushes against) and the spring cup (where the bottom of the spring sits in) are 16" apart, then your spring is compressed to a length of 16". Now, if you adjust your preload ring another inch the 2 points are now 15" apart. So there is no way your spring is still 16" because well....math.....So, to the rider the added preload will most definately be felt as a "stiffer" ride. The only way to increase ride height without changing the "feel" is to change the shock mounting positions. Either a lower upper shock mount (typically on the frame of the machine) or raising the height of the lower shock mount (typically on the A arms or trailing arms) hence what is typicly referred to as a "lift kit".
That's the thing you are wrong about. The "at rest" (vehicle weight on the suspension) spring length does not
change when adding preload. If it did then yes, you would be correct it would make it ride stiffer, but it simply cannot. Preload is "pre" loading the springs, but once you actually put the weight of the vehicle on it, the amount of preload you have doesn't affect the length of the springs. I challenge you to give it a try. What you are saying makes sense in your head, because you don't really understand how it works. Measure your springs between the preload ring and opposite perch, then add some preload and measure again. Just make sure to fully settle the suspension and move the vehicle before measuring each time. The spring length remains the same, you just have more shock shaft outside the shock.
You should add preload when you add weight because the extra weight compresses the loaded
spring farther, reducing ride height. You want to remove the preload when you remove the weight simply because I'd you don't you will have too much ride height, which reduces your available down travel.