Because of the way Rex, Cyber-Hound's second and third abilities are worded, he can continue to use abilities of exiled cards even after leaving play
Invasion of Ikoria has the same effect for only a single extra green mana, with the added benefit of being able to transform into Zilortha, Apex of Ikoria, which can allow you to defeat your opponents way more quickly
The third iteration of Loot also happens to be its best, with versatile abilities to help you do a ton of different effects over the course of a game
This turns the spell into an engine
Syndicate Heavy's last ability looks at how much life you've gained in the turn, even if it wasn't on the battlefield when you gained life
Force of Negation is no slouch in Commander, either, with nearly 350,000 decks running the card by EDHRECs current count