It nullifies damage but doesn't actually parry the enemy, as in pushing them back, creating an opening or just interrupting their attacks.
It was much more useful in DMC3, where you could parry several enemies and make them stagger backwards, allowing you to attack them while they are vulnerable.
Even Beowulf's charge attack can be stopped by a Royal Block.
I use Royal Guard a lot for chapters where I fight Arachnes because I always create an opening by parrying their scythe attacks and knocking them back with a Stinger.
Noticing the lack of stagger animations for the enemies made me realize why I didn't use Royal Guard in DMC4 as much as I did in DMC3.
It was much more useful in DMC3, where you could parry several enemies and make them stagger backwards, allowing you to attack them while they are vulnerable.
Even Beowulf's charge attack can be stopped by a Royal Block.
I use Royal Guard a lot for chapters where I fight Arachnes because I always create an opening by parrying their scythe attacks and knocking them back with a Stinger.
Noticing the lack of stagger animations for the enemies made me realize why I didn't use Royal Guard in DMC4 as much as I did in DMC3.