Instants that copy other spells usually cost (Reverberate, Doublecast, Dual Strike, etc.)
Its the backbone of various cascade decks, most notably ones leveraging either Crashing Footfalls or Living End
Pairing it with cards like Entomb lets it enter as a copy of a massive creature such as Griselbrand in Brawl or any other top-end threat your Commander deck is built around, while also exiling the original card to prevent recursion
Throwing out an Exsanguinate after a mid to late game board clear to claw back some life buffer can be game winning, even if you have to go on to finish your opponents in other ways
Instead of running X amount of card draw spells, for example, you can cut out the weakest ones and just tutor up the best ones when needed
For full details on what you can do, check out the Scryfall Syntax guide