He can walk through walls. Suddenly be standing over you in your sleep. Make shit your pants scary noises in the night. That sort of thing, without ever getting the blame.
Re-directing Lukes actions changed his normal way of thinking(mostly-except when Luke does his own thing on leaving Degoba), and guides him to overthrow the empire. If Luke follows his natural instinct he possibly would have most possibly been seduced by Dada to the darkside.
As much as i dont wanna go PT, OB-1 could have prevented Anakin from the darkside if he could have watched over his every action all the time. He may have learnt he could protect Luke better by being a spirit (or a fine wine, a beer, or an alcho pop even), without the confinement of being an ancient mortal, and stuff.
Its one of those power of suggestion things that makes no scence when you think about it, but makes perfect scence if you dont think about it at all.