Holding the boom pole above shot ensures that the optimum pickup area of the mic is directed towards the subject's mouth, and then towards the ground – which we assume won't be generating much noise. This will give you natural-sounding dialogue, which is well-isolated from the ambience of the surroundings. In addition, the boom and mic will both be well out of the actor's way, leaving them free to roam the scene and not feel distracted by the sight of it.