You can Set this card from your hand to your Spell & Trap Card Zone as a Spell Card. During the End Phase of the turn this Set card in your Spell & Trap Card Zone was destroyed by an opponent's card effect and sent to the Graveyard: Target 1 card your opponent controls (if possible); Special Summon this card from the Graveyard, and if you do, destroy that target (if any).
This card may be come more relevant with the release of Dawn of Destiny and the anti-spell/trap approach. One probability is that Silver sentinal will be run only at 1 to counter HHG and the occasional Sacred Pheonix deck. Another could be that the meta shifts away from spell/trap destruction and more ramp decks will become top teir.