There are two capacities to consider at 2,400 (with three rows) or 3,200 cards and a few colors as well
Pretty much anything looking to win through combat
(Some abilities can also target unexerted cards.) But not using them has less of a direct benefit - you dont need to defend your life points, as in MTG or Yu-Gi-Oh!, so leaving cards upright is typically only worthwhile if theyre likely to be challenged by your opponent and you need them for a future turn
Devoted Druid is an obvious choice due to the infinite mana combo, but copying cards like Orcish Bowmasters and Drannith Magistrate that arent swept away by a Wrath of God is incredibly useful
The beginning of a new year, 2021, wont fix that
However, when the dust settles and her rider claims their reward, Keleth vanishesonly to reappear on a different battlefield sometime after, searching for a new rider