Each player creates a 1/1 black Rat creature token for each 1 life they paid this way
The more value you can extract out of every instance of gaining life, the more powerful and commanding your deck is
Drannith locks commanders in the command zone, shuts off casting from the grave and exile, and so much more
Just as well, having additional protection in the creature category from cards like Mother of Runes, Sigarda, Herons Grace, Shalai, Voice of Plenty, and Giver of Runes goes a low way towards keeping your card advantage and aggressiveness consistent
Its like hes holding up a big sign that says, "Pretend I'm whoever you need me to be!" This makes it way easier to flood the board with your powerful Floodborn characters and gives you a huge economic advantage without all the waiting and wishing
Using the considered best overall cards doesnt always mean they are always the optimal for your deck