I think we're all looking for too much logic and consistency here.

If it were to make sure pax were focused on safety during takeoff and landing, you'd expect them to also turn off all IFE for the same ten minute periods.

If it were to prevent flying missles, then you'd expect them to also require that all carryon materials be stowed during the ten minute periods.

What's painfully obvious , though, is that it could truly be for any of these reasons, but no one ever bothered to apply it elsewhere. Someone in a meeting one day probably just said "We should ban xyz during takeoff and landing for abc purpose," and everyone was just tired and wanted to go home, so they approved it. Now, it gets all wrapped up in this safety flag and we can never undo it.