Jump Scare's goal is to surprise your opponents with top-end battlecruisers by manifesting them and then turning them face up at an opportune time
Since your deck will be full of instants and sorceries anyway, that prowess ability can grow it into a 4/5, 5/6, or something waaay bigger depending on how many cards you can play in a turn
Its a bit of a dice roll but can be quite spicy, especially if your deck doesnt care whether its resources are in your hand or in the graveyard
Some cards may trigger during your cleanup step - they take effect now, following the usual stack rules if multiple abilities are activated
Seriously, why is a card that is beastly like Massacre Wurm, but also sneaky like Ink-Eyes or Beacon of Unrest so affordable
Even when I open one of them, though, the section dropdown remains the same