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.
Patch 2.3.0 (13-Nov-2007): Barkskin cannot be used while under the spell effect of Cyclone.
Patch 2.1.0 (22-May-2007): Barkskin now reduces all damage taken, with its duration reduced to 12 seconds and cooldown reduced to 1 minute. The tooltip has been adjusted to indicate this ability can be used while frozen, incapacitated, or cowering in fear. That functionality was already present, but not listed in the tooltip.
Patch 2.0.3 (09-Jan-2007): Barkskin is now usable while stunned. Adjusted the tooltip for Barkskin to reflect the proper functionality.