What's the benefits of using your shotgun on a boom pole instead of just holding it in your hands? As I see many people use the boom from above, why isn't it better to position the microphone at mouth level with your bare hands?
The main point of having a handheld or boom mic is to keep it away from the face.
When filming, your focus is on the individual, so usually a shot will have face, or head, or head and shoulders, or head and torso - so you will see that the camera tends to not go above the head but can capture a lot of below-the-head images. So a mic above the head is less likely to end up in shot.
You also get the added benefit that a boom operator can position the mic appropriately - and a good operator will have a much better idea of where to position it than most people do.