Old Mobile phones are slow and run on old operating system, which makes it difficult to make it your primary mobile phone.
Lets say you have just purchased a new phone and you are not getting good offers for your old mobile phone, you may consider keeing the phone as it will have multiple uses
Mobile phones are the most powerful devices that we carry and it can have plethora of uses.
Keep it as a backup phone. Old phone can support you in case your primary phone is broken or damaged.
Use it as a dedicated camcorder. Yes. its good for Video recording and you dont have to worry about getting calls when you do video recording. You can give it to someone who only want it for videos or pictures.
Use it as a baby monitor. Install baby monitoring app and use it to monitor your child
Use it as a video doorbell. Video doorbells are costly, mobile phone can provide good alternative as Video door bell
Create a dedicated VR headset. Watch Movies using VR Gear
DIY Google Home.
Leave it on your nightstand - Alarm clock , ebook , meditation and multiple use
Dedicated e-reader: iBooks, Kindle, Nook, OverDrive -- you don't have to limit yourself to just a single app.
Meditation player: Your primary phone will have lot of distractions why not use your old phone for meditaition when you want noiseless undisturbed focus.
Spare Roku remote:
In your Car - it's loaded with offline navigation aps (Sygic, Navmii), and music from my library, so it can be a GPS and basic MP3 player for my car.