This spell gives you the opportunity to cast a spell without suffering pushback.
This spell is especially useful for casting [Tranquility] and [Hurricane], as they are very expensive, and Tranquility in particular has a long cooldown. Even though you can still be interrupted by counterspell mechanics, Barkskin will allow the full duration of the spell to take place in lieu of damaging attacks.
Every spell can have a reverse reaction from that originally intended, an example of this is the barkskin spell. Malfurion Stormrage developed a reverse effect to use against the Burning Legion. This variation of the barkskin spell also hardens the targets skin, but at the same time makes the target unable to move, turning him into a statue.
Patch 7.1.0(2016-10-25): Now learned at level 26 (was 36).