While Terra provides a continual means of milling and a mana-activated ability that brings creatures back from the graveyard, Celes only offers milling as her enter-the-battlefield effect
Whenever a permanent an opponent controls is put into a graveyard, put that card onto the battlefield under your control unless that opponent pays 3 life. Suddenly every lost permanent is a potential ally for you, and if not, your opponents have to lose life just to stop you from stealing their stuff
The racers of Aetherdrift have no intention of slowing down, and neither do we
But focusing on discard costs also allows for better use of your resources throughout the game
Its particularly good in one and two color decks, which I think is fantastic, because decks playing fewer colors should get some rewards
That led to it becoming a staple in many popular decks thanks to the ultra-powerful Badgermole Cub from the Avatar: The Last Airbender set, which doubles its mana output