To start with, the following situations:
- The question is asking for a list of examples. In this case I think it makes sense to use the voting system as a way to rank the answers, and it's easier to do this if the question is CW because then votes do not affect reputation, so they are more honestly judgments of the answer than of the answerer. Questions of this kind also often have the property that it makes sense to lower the edit barrier since different answers will correspond to different examples of some thing, and multiple users might want to contribute information about example X in the answer corresponding to example X rather than starting a new answer.
- The question is "soft" or otherwise impossible to answer decisively. This one is debatable; I guess people feel like it doesn't make sense to gain reputation on a "soft" question.