Domri, Chaos Bringers first ability alone makes it an amazing planeswalker for any creature-based deck that can run it
While exceptionally expensive, there are quite a few ways in black to generate tons of mana very easily, making it not too difficult to cast later in a game
While toughness is typically a measure of a creature's defensive ability, the printing of cards like Assault Formation, which allow creatures to assign damage equal to their toughness, has made these creatures sought after for toughness-themed decks
Often, this is because theyre creatures
The simplest option might be to just use Ashnod's Altar to turn them back into mana, use Numa to turn that mana into +1/+1 counters, and then to swing Tana again to make yet more saprolings