The attribute can ease the leveling process if you know what it does before hand.
Like Ersakoz said, what it does it’s to substitute all of your Atk stat in the stat window for the shield Defense. So, let’s say you have
1000-2000 minAtk-maxAtk after equipping a weapon + red gems
4000 Defense in the shield info window.
In this case the attribute will provide more damage with the skills that are listed for it, but like it was already mentioned, your total atk will surpass your shield def at end game if you upgrade a high lv weapon and a shield at the same levels.
This still can ease your leveling experience because it lets you to only focus on upgrading your shields and ignore the weapon while you level up, saving you resources and time until you reach end game where you can focus on a end game weapon instead.
Also, this opens some strange combinations because you can totally ignore the weapon atk and focus on getting a weapon with a nice effect. For example, you can use the 5 hammer all the way to end game, this hammer has 5 gem slots and 5 AoE ratio and is cheap to make, you could also stuff it with yellow gems (reds don’t work with shield attribute) and focus on leather armor to run around with 8 crit atk gems (5 in hammer, 3 in shield) for some big crit atk boost.