or
destroy Nasus's ability to engage, especially if you are playing a more immobile ADC
if you are a mage, you should have some sort of CC to stop Nasus from engaging on you if you were in range for W. If you dont, then you are playing a poke mage, in which case for you to be hit by W means that you are out of position, or you're morde so you can facetank Nasus (at any stage of the game) Either way the AS slow means nothing to you, your DPS is still the same.
If you are a support or tank, then thank god he hit you with W, now your teammates are free to fire on Nasus.
Again this is all in a teamfight scenario where you have a frontline to engage/peel the Nasus away. If you are a lone ADC who gets caught by a righteous glory nasus then you honestly deserve the QQ that he is about to hand you