Well, the possibilities are endless ;-)
For example blocking a corridor which you may want to unblock later without having to destroy everything in sight; or hide behind the barrel from enemy's fire if you need to reload or change armor; or hide behind it, then approach an enemy oblivious to your presence by pushing the barrel right nex to him, then jump from out of it and decapitate him with your chainsaw - definitely useful for melee characters.
Though I must admit that bottling a hell baron with exploding barrels and then blowing them all up has its charms too, heh heh :-D