High parry shields are good for all purposes. It has a high rate to parry incoming attacks to reduce damage taken.
But parry shields usually have lower Block (damage reduction), so while you can frequently parry attacks, it doesn't block as much damage.
High block shields are the reverse - doesn't parry often but when they do, it reduces significantly more damage.

High block/low parry rate shields are good when you use your skill Bulwark to increase block rate (+60% 15secs) to take on a big hit/nuke from a Boss.
Cooldown on Bulwark is 180secs so you'll find yourself mainly using it for big pulls or saving it for known boss attacks.