If a spell you're casting with escape has an additional cost of discarding cards or sacrificing permanents, you may exile cards discarded or sacrificed this way to pay that part of its escape cost
Most of where you see it getting used in cube these days is in combo decks where a sacrifice outlet is required or there are loads of graveyard synergies going on
You can also sacrifice them from the board, which is something else this deck excels at
Blood Moon used to be a great way to stop Urzas Saga
This was revealed in the mechanics article for the set, released quietly around the time of the debut
The value of cards on Arena is determined by their rarity on the client, meaning that one of the more powerful planeswalkers and mythics on Arena in Tezzeret, Cruel Captain, has an identical value to a card like Kylox's Voltstrider, another mythic that sees absolutely no play in any formats