Because covering the purge button during donation can be a common mistake, divers should learn at least three basic ways of clearing the second stage of water. Purging with the button is the best method because it will clear debris out of the mouthpiece if any had entered. Humming or otherwise exhaling into the regulator will work if the purge button is accidentally covered. Finally, a third method involves using the tongue as a piston to force the water out of the mouthpiece like a pump or a plunger if the diver is unable to adequately exhale.
The advantage of donating a regulator from the mouth is that you are providing a working regulator that won't be clogged with debris, and as Lynne said, it makes it easier for the OOG diver to purge the regulator for added safety ensuring the mouthpiece is debris free. While DIR training advocates this method of keeping the second stage free, it has always been a universal precaution.