Anecdotal for sure but I had it explained to me that it was so people didn't have their faces in their phones/devices during take off and landing, those being the most dangerous periods of a flight. The claim was that they want to ensure the passengers are attentive and able to hear any emergency announcements in case there are any issues.

I got in to this conversation with a flight attendant who insisted during take off that I turned off my mp3 player (quite a few years ago). The device had no wireless capabilities at all so there was never any chance of it causing any RF interference which I explained. I was in a seat next to the emergency door at one of the wings and they said they typically don't bother enforcing the rule for most seats except those directly at an exit. Again, anecdotal but it sounds a reasonable enough explanation and a fair request for those seats at emergency exits.