I can imagine a few reasons :
- to lower the risk of electro magnetic interference (although very limited !)
- to make sure people pay attention to what's happening
- to make sure everything is put away (imagine people with laptops on their laps etc...)

<rant>I am too very annoyed when you see people blatantly ignoring CC instructions … having the CC going in the cabin telling people off is just a waste of time and distracting them from any potential safety issue </rant>