Longer answer: Coven is an ability word , and as such it varies from card to card
26, Feb
Since most competitive decks were playing four copies each of Professor Oak (Professors Research without the Supporter limitation) and Bill (draw two cards), players were going through their decks at extremely accelerated rates
Creatures that bounce on entry, like Cavern Harpy, are also great fits here, nabbing you extra value while you recycle your enters and leaves triggers
And bouncing has a lot of flexibility as a way to survive removal or blocks, adding a useful tempo element, reloading a creature with a powerful enters-the-battlefield trigger, dealing with auras and so much more
Once Ulamog has entered the field, it can truly devastate an opponents whole board thanks to the best Annihilation effect ever printed on an Eldrazi card