Drawing cards from the bottom of your library is exactly the same as drawing from the top, except that you take the bottom card of your library and put it into your hand
In graveyard decks, that second card is not always lost. Sometimes it is just moving to its second office
For five mana, this Sorcery destroys all non-Black Creatures, dealing one damage to you for each Creature destroyed this way, plus three The mana cost is reasonable, but being triple Black makes it harder to cast
While taking a card from your opponent's hand is great, targeted discard give you the additional value of seeing your opponent's entire hand, which allows you to play more efficiently, since you know which cards your opponent is likely to play over the coming turns
Paying five mana instead of nine, ten, or more has obvious upsides
When I have enough decent targets, I want Gearhulk, but a little too often I find I am not there