They tell you to switch off the device's broadcast functionality ("airplane mode"). Something they don't need for navigating with an iPad as it's just offline maps / apps).
I haven't had a captain tell me to turn off electronic devices for years. The flight attendant asks me to remove headphones for the safety briefing, which is reasonable, and we're told to put it into airplane mode for takeoff/landing
I accidentally left my iPad in the seat-back on a recent flight — my case, it turns out, is a perfect match for Delta-leather-blue. Even though this is an iPad with cellular, I'd switched it to airplane mode and thus was unable to locate it with "Find My". After three days the airline hadn't found it, and I chalked it up to an expensive lesson learned.
On day four, I received a push message (from Find My) that the iPad had come to life. An airport staffer had found it and charged it up. Presumably it cold boots with airplane mode switched off, so it connected to the cellular network, received the "lost device" command, displayed my contact info, etc. I received a call shortly thereafter.
When I picked up the iPad at baggage services, I chatted with the staff a bit. They let me in on the "secret" that most airline employees never switch their devices into airplane mode anymore, and for this exact reason.
While it's good to avoid EM interference from electronic devices, I think the real reason they want you to switch of these devices is to make sure you're not totally distracted during take-off or landing, and that you're not using headphones, so you'll be aware in case of accidents/emergencies.
Nah, they haven't asked passengers to turn off devices in years. They just want them placed on airplane mode so they're not connecting to cellular networks. Bluetooth and WiFi are just fine.
The pilots’ devices are selected to airplane mode as well. Many carriers do allow WiFi to be turned on and have a separate WiFi network onboard that allows for operational access. Mainly weather.