Cooking skill increased by 10.
Prior to an undocumented change in Patch 5.2, this item was the only one in the set that had no binding restriction. Thus it wasn't unheard of to see this on the Auction House.
To my knowledge, it was changed so that it would be consistent with the others. It ended up being just as well, because cross-server mail for heirlooms was finally possible in Patch 5.4.2.
Note: This tabard does not count towards any achievements.