I read this post this morning and you got my curious. I just so happened to have a flight this aftternoon, so I asked one of the flight attendants.
Her explanation:
The "Cleared for departure" announcement appears to be mostly a communication between the crew members. Doorbell chime from the pilots is telling the flight attendants: "It's almost our turn. Are you guys ready to go in the back?" "We've been cleared for departure" announcement from the flight attendants lets the pilots know: "We are all in our seats. Everything in the cabin is good. We are ready to go." She explained that "Cleared for departure" or not... the pilots are not going to takeoff until they hear that announcement from the flight attendants. All the years of flying and I had never thought about any of that until I read your post!
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