Obviously, by definition, a stereo source can be plugged into a stereo channel. However, because I've never had a reason do that, I can't see the need to plug in stereo sources as common enough to warrant the number of stereo channels on so many mixers.
So I assume that there are uses for these channels that I am just not seeing (or I'm too narrow sighted to see the usefulness of being able to plug a stereo source into a single channel of a mixer).
What are the most common uses for a mixer's stereo channels?
This question also applies to stereo effect returns (which I also can't see a common use for because I neither use stereo effects nor notice others often using them).
Edit: Some of the answers and comments seem to be taking umbrage at the second sentence of the first paragraph. The sentence was not meant to suggest that I think that no valid use cases exist outside of mine. I recognize that my usage is keeping me from seeing other use cases, which is a shortcoming of my own and the reason that I posted this question. I apologize for inadvertently wording the question so provocatively.
Edit 2: I titled this question poorly (I'm hesitant to change it now since it has got so many views), but the intent was to ask "What are the benefits of a stereo mixer channel over 2 mono mixer channels?" I seem to have thought that simply asking for a list of uses for stereo channels would make those benefits clear, but it's probably more helpful to receive answers to the underlying question directly.