Obviously I can't know for a fact. I also thought they likely analyze photos and that's my chief suspect even now. But critical thinking demands to keep an open mind -- and that's how I suspect they might use fully-charged iDevices (that are expected to sit on the charger for several more hours) for unsolicited and non-advertised P2P computations.
I don't claim it, I merely suspect it.
Monitor your network traffic.
Lots of people do this already for various reasons and haven't noticed anything like this going on.
You seem to have an axe to grind here and I refuse to participate. I even objectively admitted that I can't know and that I only suspect yet you (a) offer a non-solution and (b) likely downvote a comment because you don't like it.
Sheesh. Fine. You do you.
Do you have any evidence of this, or are you just making up something that sounds good to you?
More likely it's doing the background on-device work that Apple has actively advertised for a long time, like analyzing photos to attach metadata to them. (There's a lot more to this than is obvious at first glance - I can text "penguin" or "sunset" in my Photos.app library and get a set of matching pictures back, for example.)