USB OTG on android phones is severely underrated.

• I can plug in a USB drive and transfer files around, I've used this to manage my retro handheld SD cards before.
• You can tether your hotspot over Ethernet to your computer with an Ethernet adapter.
• You can plug Ethernet into your phone to get faster connections.
• You can plug a mouse into your phone and get a cursor on screen. Not super useful tbh, but kinda cool.
• You can use your phone as an external webcam for your computer.
• It's a bit more annoying than it used to be but you can use your phone as a universal IR remote with a small adapter and free apps (I miss my built in IR blaster from my S3).
• I haven't used it much, but I can plug in a RTL-SDR dongle and get aerial TV on my phone, or a radio spectrum analyzer. I used it to discover that my garage remote is about to die and that's why my car's garage button won't learn the signal.
• USB (or Bluetooth) game controllers just work.

Definitely a relatively niche usecase but I have SSH clients, terminal apps, RDP remote access clients, and other networking tools as apps on my phone for quickly messing with things. Very helpful to not need to bring out the PC when I'm fixing my network.

The ability to VPN into my home network to access my NAS. Honestly being able to access my NAS in general is already great for backups or just so I don't have to think about what's physically on my phone.

With a cheap Bluetooth device I can connect to my car's diagnostic port (ODBII) and check engine codes. No more trips to the mechanic just to get it diagnosed.

WiFi direct cameras are a great addon too. I have a wifi endoscope (camera on a long bendy stick) for inspecting inside walls and my phone works as a screen for it.