Ghalta and Mavren is an exception because it combines the abilities of both legendary creatures
Add in the number of useful enchantments which can easily be cycled (including Cast Out, Shark Typhoon, and Decree of Silence), and you can appreciate the kind of power Replenish unlocks
Generic creature type payoffs like Banner of Kinship, Herald's Horn, and Urza's Incubator can also have major benefits
Sometimes, this upgrade in damage will be what you need
Previous double-faced cards like those in the Innistrad sets can only be played on the front side, and then transform to the back side when specified conditions are met
Ichiga, Who Topples Oaks Legendary Creature Spirit Trample Remove a ki counter from Ichiga: Target creature gets +2/+2 until end of turn