Most old phones have poor battery performances. However, here the phone is almost always charged and a single app that doesn't require a fast CPU is running.
Can show time, weather, make IP video calls, tell you about your daily schedule, has an assistant... so it's basically like a Nest Hub but with more possible features.
Similar to some physical password vaults, like Password Vault by Hammacher Schlemmer, it's safer than most cold storages because if someone gets your device it's harder to brute force both encryptions than opening most vaults. Also you likely have time to revoke all existing tokens/passwords before the attacker.
Storing for example OTP back-up codes in a password manager connected to the internet (your home computer, LastPass, 1Password...) is somewhat secure but you always risk that a software exploit may give an attacker access to your accounts/data. An offline storage, however, may not be compromised as there is no attack surface. You only risk something if someone grabbed your device, but even then you are likely capable of revoking any password/token stored on that device very quickly.
You cannot copy/paste or fill forms that way (as the device is offline). So it's not viable to store commonly used passwords. It's only good to store rarely used critical data, like: OTP back-up codes, cryptocurrencies, recovery keys and such.
As annoying as ads can be, these are people paying, hoping to bias your mind. If you don't hear or see the ads, you're basically exploiting the people paying for it. Also remember to deduce your electricity usage.
This is known as a "Perk farm" - when you have multiple devices (up to 5 are allowed on a single account) all running Perk at the same time. Using 5 devices constantly streaming ads can yield somewhere around $50/month in the US (not as much in other countries).
Once you set up Perk on your old phone, you can leave it there and never touch it again. It will play ads, and you will earn Perk Points which you can redeem for gift cards, pre-paid Visa cards, or Amazon gift cards.
America has the highest rewards per ad - you get 4 Points for every 2 ads you watch. Canada is next up with only half as many points - 4 Points for every 4 ads. The story gets much worse for all other countries: 4 Points for every 20 videos.
Phones with Amoled displays are prone to burn in when used excessively, so their screens may be ruined if you stream ads on your phone all day.
Streaming ads will also put strain on your internet, and if you have a datacap it could fill up that cap.
Some common sensors (exploitable directly for example using the Home Assistant app) include:
Camera
Light sensor
Accelerometer (vibration/motion sensor)
Sound sensor and emitter
Temperature and pressure
Proximity
NFC reader
GPS and compass position
Dead phones, why not. However why throw away a working device that took resources to build and people could gladly use. It feels like a disrespectful thing to do.