It works along with block placement. Generally, you would be able to use sleep vials to put them in sections of the map where they might be blocked off, or at the very will impede their navigation (say you want to get a healing critter away from a mob). Situation specific, you would be able to move monsters away from status effects that could have boosted them( shock gels away from shock traps), and you could drop some monsters into certain traps, i.e., undead onto the path wheel barrows.
They also act as a short-lasting movable barricade (when dropped), since a sleeping monsters blocks the path for any other nearby monster. Or you could be mischievous and drop them onto other players, I suppose ... :3
lol Carry sleeping players...that would be hilarious and useful at the same time.
AFAIK, most bosses can't be put to sleep? Don't know for sure, never really tried it...but anyway, what exactly would be the use in relocating monsters? Players who are AFK is useful, but I don't see much use in the monsters being carried around.
~Sev