I'm not done yet
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
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
This means that you can trade in a rare wildcard for any available rare card, an uncommon wildcard for any available uncommon card, etc
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
Longer answer: Coven is an ability word , and as such it varies from card to card