rear differential flush on 2011 Honda Pilot

Whydoes the differential need to be flushed at 15000 miles

Asked by for the 2011 Honda Pilot
honda states that if you live in mountainous areas or tow regularly, the first diff service should be done at 7500
miles and every 15000 miles thereafter. it is NOT a flush, it is drain and fill. they say the fluid can stress or shear. with any new vehicle still in warranty, it is best
to do just as they say. if there was an issue, at least they can't decline a warranty repair because of lack of