If you have an extra wifi router as well, secured network-security cameras. Lots of programs out there to do just that (been looking at a few and I'm not 100% on a perfect recommendation till I set something up myself).

I use my phone for banking, paying, made appointment for health checkup, route planning and gps, bike computer/gps, buying stuff, diagnose car and look for more info, check for sound decibel, check the weather, visit some questionable site made for adult, save and manage password, take photo(as note making), make memes, switch on/off my table lamp and ventilation fan, order my slave robot vacuum to do their work, and pay all the bills.

Constellation tracker and identification!

Camera and accelerometer can let you do a lot of really creative things! I've tried a few projects but the main bottle neck is battery so it has to be some inside use that can be wired with power. I saw some guy stick it to a dryer with an app that notifies when dryer is done.

Rejecting calls

Forgetting to reply to messages

Ignoring emails

Writing comments then deleting the text without posting

Unlocking your device only to immediately forget why you needed to check it.

Theyre essentially the swiss army knife of tools:

• Flashlight
• Camera
• Level
• Calculator
• Phone

Map your local area.

Use StreetComplete or SCEE to fix parts of OpenStreetMap data in your area. Fun to do when walking around areas you know.

Or use a higher level editor to add missing paths, services and buildings.

I’m an independent contractor, and I basically run all of my business from my phone. Aside from making calls and sending texts, I have templates in Google Docs that I can edit and then email out as quotes and invoices. I keep spreadsheets of my inventory. I scan into Notes the repair slips so I can keep a copy. I use the navigation apps to route me to my stops during the day. I have a template that I edit to create my timesheet to submit and get paid.

I run almost my whole business off of a small handheld phone, something that was unimaginable just a few years ago.

Anything's a dildo if your brave enough. Plus, it vibrates!

I use my old phones that still work as media players, I uninstall almost everything and basically only use VLC on them to watch stuff on my NAS. They're like tiny TV's scattered around the house.

Now I just only need to learn how to broadcast locally from the PC so they can play the same thing at the same time. I know VLC can do it because I've seen dozens of tutorials but they all must be missing something because it never worked for me.

I started looking into cozy games on my phone so anytime I get the urge to doomscroll I turn to that instead.

https://phyphox.org/

Make phone calls.

i have a drawer which would otherwise have been empty, but thankfully i have a nexus 6p, a pixel 2, an lg q6, some lenovo phablet, and a galaxy note 5 to use up that space.

they also do make mighty fine paperweights if one is needed in a pinch.

Reading. Books are super easy to ahem find. OLED screens make reading really comfortable at night. Black background, dark orange text, and turn off all the lights and it's like text is floating in the air in front of you. There are plenty of epub readers out there. Moonreader is my favorite. I paid $5 for it years and years ago now. Absolutely worth it.

Not sure if "good" is the right word, but at least cool.

Torrenting, high speed mobile data modem (especially with manual selection of frequency bands on MediaTek), local OpenSpeedTest server (available as app), WiFi analyzer (most used channels), VNC client, the slowest x86 emulation in Qemu-based Limbo PC emulator, SDR receiver software (SDR++, SDRAngel, Welle.io, dump1090, SatDump), RTL-TCP server, SSTV decoder and encoder, HTTP proxy server, Kiwix server, NGINX web server/proxy, Navidrome server, Cloudflare proxy client, SSH server, VNC server (only for Termux's desktop), satellite tracker, Mifare Magic NFC card programmer (MCT), audio spectrum analyzer, serial terminal.

I wanted to attach screenshots, but realized it's way too much stuff.

I use it for a lot, but one I haven't seen mentioned. I use it to support my ham radio hobby. I have a satellite tracker for when I want to contact radio sats, a solar weather app for checking HF propagation and I have echolink which let's me connect to hundreds of radio repeaters around the globe.

*HF = high frequency, its a section of radio frequencies that bounce off the atmosphere. Let's you talk worldwide if you have the right frequency and conditions. Solar weather significantly impacts how radio waves interact with the upper atmosphere.

Some of my favourite mobile centric uses (I'm a FOSS leaning Android):

• I like to try to ensure most things are available offline: maps, notes, passwords (manager also holds "emergency" documents), media, ebooks, podcasts etc
• OsmAnd has offline Wiki articles, this is awesome when travelling
• OsmAnd can be great for finding POI's such as food outlets, toilets etc when travelling (I since extensively mapped my own locality to help visitors by way of thanks)
• Using stuff I self host synced to various devices: Nextcloud, Joplin, Paperless-ngx, Immich, Jellyfin & a bunch of others
• whoBIRD is great especially when travelling
• If WiFi/data is unavailable when travelling away from home, hook the phone up to TV with a hub, HDMI, keyboard with track pad & it becomes a full media system

Calling your loved ones

Lots of people gave good uses here so i'll give one too. the other day I lost my fitbit and I didn't know wtf I lost it then I remembered smartphones have bluetooth and emf sensors so i downloaded an app to find my fitbit and I found it. Felt like I was going mad looking for it lol

It's a music player, e-reader, and mobile videogame platform that can emulate any retro system and has unique games based on physical activity and geolocation.

It can also take pictures and send IMs, I guess.

A unified remote\console for displays, ACs, PCs and whatever

On-hand manuals and checklists/

Podcasts\books player

I use mine a lot for geocaching. The Seek app by iNaturalist is also pretty good

I love emulating old Gameboy games on my phone. It can play things all the way up to Switch, but there's sort of a nice mix of nostalgia and simplicity to just go monotone. No micro transactions, no server connecting, nothing. Just me and the bits.

I guess that's not terribly beneficial, unless you count my mental health.

I have a lot of emulators on mine I recommend lemuroid as a gateway app it does most older systems and many of the arcade machines of my youth. Assuming you legally own the roms of course.

I find a cheap Bluetooth controller works a lot better than the touchscreen though.

I have a tuner app, drum machine, and recording apps on my phone. I like to pretend I can play trumpet, mandolin, piano, hammered dulcimer... which means I practice something nearly every day.

GPS

I used an old phone one time as a light sensor in my grow tent. The sensor is close enough to tell me how bright the tent was.

Play a compilation of song covers at exactly 135 BPM so I can do my treadmill without losing my center or my step.

Work the laundry machines, and set a separate timer because the app notifications are unreliable.

Text to check that my kids are alive and assure them that I and their dad are also not dead today.

Order grocery delivery.

Listen to birds and learn their names (Merlin)

Be part of the earthquake alert system (MyShake)

Wordle/ Quordle/ Waffle

Mine's pretty great at reading a 1400-page manual for an 8-bit system. Whether or not my habit of reading a 1400-page manual for an 8-bit system is actually beneficial is up for debate.

Calculator, flashlight, camera, blunt object...