Most importantly, giving your demon deck that would otherwise just be heavy on black access to blue is bound to be useful, giving it access to stronger draw spells and counterspells
Mardu Woe-Reaper is surprisingly strong since a warrior type creature that only costs 1-mana and is a 2/1 is pretty insane
Rakdos, Patron of Chaos was released in the Murders At Karlov Manor set and Rakdos, the Muscle came out in the Outlaws of Thunder Junction set
Each time you sacrifice something, Korvold grows bigger and replaces itself with a card, so your board keeps moving while your hand stays full
It can be pretty handy in food-themed decks
Eldrazi are one of the most popular creature types in Magic, so there's always going to be some kind of value to them