It has two steps: End step : The last chance for players to take actions, such as casting an instant spell
Darksteel Splicer Seven mana for 10 power and toughness across three bodies is pretty great value on its own, but Darksteel Splicer takes it even further by rewarding you for playing additional Phyrexians
If you pick Clara Oswald as your Doctors companion, you can add black to your color identity and run some sweet Food synergies, including historic ones like Gyome, Master Chef
Protection from Humans
It slows down decks that try to cast multiple spells in one turn like Storm, Tron, or Cascade, and it also messes with greedy mana production
The same is true of the delve mechanic and the Phyrexian mana mechanic, and while neither of these mechanics ended up needing to be banned while in Standard, both are right on the border, and one slight miss could lead to major problems and bannable cards