last edited by Mike Noble
Tormod's Crypt is for removing Cabal Therapy(s) from their graveyard at the last moment of priority before they're able to cast it. There are three good, but not foolproof, heuristics that I use:
If they're showing Cabal Therapy and have a creature on the stack (return trigger or being cast), blow it.
If they have a creature in play and are activating Bazaar with a Dredge card in the yard during their main phase, blow it.
If they have both a creature and a Cabal Therapy during a step/phase where they can't cast it (usually the draw step), blow it.
In all these cases, that's the last moment you'll have priority before they can cast Cabal Therapy. The popular configuration of Pitch Dredge is a slow, disruption-heavy build that has Nacromoeba, Ichorid, Prized Amalgam, and Hollow One as free creatures. Avoiding Cabal Therapy will help you avoid them pushing your critical turn back.
This is basically what @Vindicator said, but with the heuristics spelled out.