Its not so much to lessen the strain on the starter-box, even though it will.
It's so that the engine can 'pop' over a little easier because the compression is less. As normal, the p/s is really tight which makes getting it to run the first time is usually a real PITA! So adding a shim and taking out the plug a few turns makes it a little easier to start until its up and running, then you can tighten the plug and after a few tanks (once the pinch becomes less tight) you can then remove that extra shim and run where it should be running.