You can turn an old phone or laptop into a wifi extender.

You can use an old iPhone to get iMessage working on an android.

If you attach a lens to it you can use your phone as a binocular

You can use your phone to scan a document and turn it into a digital file.

You can use it to make a signature, upload to computer to make a legally binding e-signature (the security of this is ambiguous, do at your own risk).

Theres probably an app somewhere to use it as a trackpad (if not can somebody please do this).

I'm pretty sure there's an app to use it as a secondary monitor for your PC or laptop

If you ever repair or fix things, take pictures at various points to have a reference to how it comes back together.

It you ever need to remember anything just take a picture. (Use this all the time at work for stocking things, take a picture of its location address, walk down the warehouse etc)

I think there's an app to use it as an angle gauge by tilting it. Used for hanging pictures and stuff.

Probably a pair of calipers with the right app and calibration.

You can get a "remote controlled button presser and switch flicker". You attach it to a light switch or anything else to remotely activate it. Great for turning on and off a space heater or computer.

Use an old one as a remotely viewable security camera, with battery.

Pierce the battery to make a fire (can only be used once)

That's about all I can think of right now