If an instant or sorcery card is put into your graveyard during your turn, you'll be able to cast it right away if it's legal to do so, before any player may attempt to remove that card from your graveyard
He can catch players by surprise by overwhelming them with strong cards, but he's still pretty vulnerable to being rushed down and removal effects
As Verhey explains in his video, it's very handy for Goad to appear regularly in Commander games, for a number of reasons
Its a pity you have to sacrifice it and the Land search is not a free ability, but it is still a must-have in multi-coloured decks.
These will transform after you fulfill some kind of condition on the card, usually either an activated ability (i.e., paying to exile Terra, Magical Adept) or triggered ability (i.e., having six permanents in the yard for Exdeath, Void Warlock at end of turn)