I know if I toggle Airplane Mode, it will turn wifi and bluetooth on and off-- but I can manually toggle them. On an iPhone or cellular-enabled iPad, airplane mode kills the cell radio. But there's no cell radio in my wifi-only iPad! So what does airplane mode do for me?
It's a feature kept on iOS so that you can quickly turn off both Wifi and Bluetooth.
IIRC Airplane Mode will power down all radio circuitry. While you may not be using WiFi right now, the radio antenna is still live and potentially responding to background activity. I'll try to find some reference material with more details.
Until recently, commercial aircraft use radio systems which are susceptible to interference. For example, a helicopter coming in to land at night can click their 'click to talk' button on their radio and airports that support this will adjust the landing strip lighting (low, medium, high, repeat).
This is a similar principle to if you leave your phone next to a public announcement (PA) system or other microphone - you hear the occasional radio chatter over the speakers.
You may want to conserve power while using apps that don't need a constantly on network connection, such as Kindle.app.