A bonus to a kill spell can be the difference between a foe focusing on your most valuable Merfolk (the MVM, if you will) to changing their mind and destroying one of your other opponents' critters instead
GX episode 157: "The Approaching Threat
The jankier players among us might even try to combine it with Teferi, Who Savors Sunsets (or whatever he's called) to get multiple activations
Another factor to look for is whether the card is holographic
But even with over 1,000 chapters in the manga, how long until the well of characters and plot points runs dry
Valley Rotcaller One generic, one black creature Squirrel Warlock 1/3 Rare Menace Whenever Valley Rotcaller attacks, each opponent loses X life you gain X life, where X is the number of other Squirrels, Bats, Lizards, and Rats you control