Basically the purpose of the settings is that the taller they are the less intensity they need..if you can't move your light higher up as they grow lowering the intensity can help..
but I personally can move the light so I keep it low as they grow but don't wanna burn Themis I constantly check it. when they flower (I believe) they need the most light in the 12 hour period..
but truth be told I ran mine at the max my bulb could take
600w for the mh and then 400w for the hps
another use for the setting is for different lights.. my ballast has setting for 400w, 600w, and super 600w so my theory is that's 50% 75% and 100% and I can use all of those bulbs at the right setting..