Cull the Herd is a sacrifice proc that trades one ally on the field for another in your deck. It gets the most mileage with powerful Slaves like Scrivener or Rat Catcher, who can effectively turn this from a 1-for-1 trade into a net positive, especially with an Overseer activating them. It's very cheap, but offers no proper field advantage; the power it holds will come entirely from the other parts of the Daramek recursion engine, and if you have no rituals, Overseers, or powerful Slaves to work with, you'll find it an underwhelming effect.
As a sacrifice proc, Cull appreciates the bonuses from Sacrificial Dais and Shrine of Rebirth. It also desperately wants an effective target to swap for with powerful activated effects, of which Slaves like Scrivener and Rat Catcher are excellent examples. Since the nature of Cull means that it's a two-card play (first Cull, then the Slave it draws), it also works as a way to extend Ritual combos, with the added bonus that it doesn't empty your deck of allies nearly as quickly as Feast or Repopulate, should you have more recursion effects to play.
Cull's weakness is mainly in that it requires other combo pieces to get work done: field presence and allies in the deck, which means it's a terrible first buy It also works poorly with most Herds, despite the name, because they don't natively carry comes-into-play effects.