It took me a really long time to "get" it. But it looks like a handy solution if you always cut sheetgoods of the same thickness, and want to reset it after a blade change or cutting an occasional door .
I pretty much use the same blade until it needs to be resharpened or replaced, don't swap between blades So as long as I'm using the guide rail I don't have to reset anything. It saves me the trouble of having to remember to add 5mm to every depth.