It's a slottable passive. There may be a rare situation that a Sorc Tank uses it to mitigate an awfully large blast. But their first mistake was playing Sorc Tank.

You see, the original Bound Aegis was very popular but ZOS nuked it into this.. worthless ability. In general, there are a few of these spells in the game that just need to be revisited because of bad design choices a long time ago.

It's counterpart, Bound Armaments works similar to that one Nightblade ability and is actually quite good. Not only as a damage spell to use but also a potent passive.

On tank builds it makes you immortal making Sorc Tank among tankiest of all.
On a Sorc DD you get that enraged Llothis cone, pop bound aegis - boom you are alive, any other DD would be dead.
As a matter of fact just yesterday a Sorc DD in my Kynes group survived triple meteors on Kynes Vrol HM thanks to Bound Aegis, while a Necro who was in the same stack of 3 meteors died

I use it regularly in vet trials. There is only question, if on front side as stable buff or back side as bad cause scenario buff.
It is also must to have for solo dungeon raiding.