Ordinarily, this means that you have to play this card on your turn, being a Sorcery, and be able to use whatever card it is you steal from that opponent’s hand on that same turn. Only by casting it do you actually both benefit from the use of that card and deprive your opponent of being able to use it from their hand. However, there are ways, especially in Commander, to turn sorceries into instants. So, there are probably some silly shenanigans you can pull off with this in the right sort of decks.
It’s not a valuable rare by any means, but it can do some neat tricks. Being able to steal a player’s spell and be able to use it while depriving that player of a card is a fairly useful effect, if not extremely situational. It’s not one of the best cards in Prophecy, by any stretch, but it has some minor combo potential.