Sludge Monster From a flavor perspective, Id like this to be an Ooze, but I get why its not
You can use them to set up cards that reanimate your equipment or Aerith, Last Ancient, but filtering the top of your deck is their main purpose
The first time you draft Sultai Delve or Mardu Warriors, you might, to some extent, be muddling your way through
Its also a binary condition you either meet that number or you dont, and having four or five unique power values on the board is only relevant as a bit of insurance (so you wont suddenly lose access to coven after one removal spell)
This makes it easy to trigger Brimazs proliferate ability each turn, especially if you build in ways to sacrifice your Phyrexians
These are one-time effects that WotC opts for when they want permanent cards that have more than one option