This is very useful against bosses such as the high dragon (though he/she can still be grabbed) and Meredith (her area-of-effect stun will not work on its user), as it makes its user a more effective tank. Though there are other shields having much higher armor than this, the invulnerability to knockback is worth the gap.
If Aveline is equipped with this shield, bumping her through impassable obstacles like a flaming barricade will not knock her back (thus, proving the shield's ability). Instead, she always screams in pain. As for Hawke, though he/she cannot also be pushed back, he/she will not make any sound that he/she is hurt. ps3