Using the heroic version (870), the armour rating of 1144 translates into 13.4% DR at 110, lasting 30s. Armour is usually ranked highly for tanks, so this partially accounts for the ranking for physical damage. Most of the other current trinkets don't give armour, and the 30s duration is long as trinkets effects go. By comparison, Feverish Carapace from Goroth lasts 10 s but is a chance on hit (though the amount of armour is much higher),
You also get an effective health increase of 555, 984 on a 2 min cooldown; effectively over 1.1 M bonus health over the course of a boss fight. Effective health is especially valuable for magic damage as it gives healers a little breathing room after large hits. Given magic damage reduction abilities are generally on long cooldowns, this gives you another cooldown. Of course, it's also useful for the same reason for physical damage (but all tanks at least have passive mitigation through armour, so it's comparatively more of a "sugar on top" bonus).
Taken together, the effects are a nice survivability package that you can use when you need it most.