There were no good answers to a Frenetic Efreet, only answers that worked half the time, so as long as you weren't far enough behind that he was ignorable the Efreet was always effective
They allow you to get back cards like Time WarpTime Warp to keep the extra turn train running
If you play 40 lands and 10 mana ramp effects, that means half of your deck is mana, leaving little room for the other stuff
It is an artifact, which means it can be Whirred for, can make Blue mana with Urza, and power up Urzas Saga Constructs
Absolute Virtue is one of the game's most infamous endgame bosses, who famously took players literal days to beat in its earlier incarnations
It's in a massive 119,930 decks according to EDHREC.com - getting that in your Command Zone to build around in an abusive color for activated and triggered abilities