It makes it difficult for your opponents to permanently remove your best creatures, and you can double dip on powerful ETB effects when your creatures die
Transform cards become even more cumbersome when their flip condition is like threading the needle
Creatures like Piper of the Swarm, Totentanz, Swarm Piper, and Ogre Slumlord can ensure your field is consistently full of rats, and the nonlegendary Piper even lets you use them to steal your opponents creatures
While a little confusing on paper, this boils down to allowing the repeated use of exhaust abilities, turning them from a once-per-game affair into a once-per-turn situation, allowing for many of the more powerful abilities using this keyword to be activated again and again
This ability is extremely powerful, and with black/white caring about sacrificing permanents, this becomes an incredibly premium card for that deck