SSH, and also when you're on the road and want to write a simple Python script to process something, or do something with your sensor data logging. Termux has a Python API to access sensor data, it has numpy, it has requests, so you can do a lot.
For me ssh to access my main Emacs session is a big part of it but I also run some shell scripts and CLI tools and services written in Go. ssh-ing back into the phone for file transfer is another important use.