It gives roughly 11% worth of mastery for WW monks for an hour. Not sure about the other specs, but for WW monks that's literally 11% increased damage for 1 hour. Could be useful for raiding on a boss your guild is stuck at or to increase the speed of heroic/mythic dungeons. Can also be pretty damn useful for the withered training scenario.
You can get back to the Peak of Serenity by going right as you descent the stairs past the first gate in the class order hall. There'll be a portal there.