Just as Burn struggles to win with Lightning Bolt alone, you need a cheap creature that generates value over time in order to really maximize your mill count, and the Crab is simply best-in-class
So, if you combine First Day of Class and one of those creatures with a sacrifice outlet, the Persist creature can just be sacrificed as many times in a turn as you want
Because of their concentration in Black, they rock a lot of Black-centric abilities, and in particular, they kill stuff
The primary exception is Amonkhet, a plane where every angel is male as depicted on cards like Angel of Sanctions
Ornithopter is a great example of one of these creatures and so is Goblin Rabblemaster
Jalira, Master Polymorphist $1 Polymorph has seen competitive play from time to time, but this is much worse because you have to let it sit around for a turn before you can activate it