The reducers property both creates an action creator function and responds to that action in the slice reducer. The extraReducers allows you to respond to an action in your slice reducer but does not create an action creator function.
You will use reducers most of the time.
You would use extraReducers when you are dealing with an action that you have already defined somewhere else. The most common examples are responding to a createAsyncThunk action and responding to an action from another slice.