Yeah, that's a valid question. I do use it on a phone and I don't think a phone's on-screen keyboard is suited for programming or heavy terminal use, but I've used termux for small things that I need to do when away from home. For example, rsyncing music from a server to the phone so I can listen to it, or fixing the config file in RetroArch with Vim, since sometimes fiddling with options can get it into a state where it needs an external text editor to fix, and Vim is easy to use on a restricted keyboard.