Depends on how performant this VM is especially if it can utilize the phones GPU well. If the GPU passes into the VM well then that opens up a lot

You get access to desktop Linux applications which can be very good. May really enhance Android devices ability to be a laptop replacement. My personal laptop is an Ultrabook from 2017. Practically every phone released these days are more powerful than it

Like say if you had a video that was 1920x1080 but the actually something by something. If you googled how to detect what the dimensions inside the black border are and crop them, you'd probably find ffmpeg commands to run and crop it. With the VM just run the ffmpeg commands

Maybe it's a really good VM and you could use desktop Linux applications well. Now you can get access to desktop Linux Davinci Resolve, Krita, Ardour, Audacity, etc. Last testimonials I've seen is that GUI applications don't work yet but that's a work in progress

If you're a software developer now your phone can conceivably be solid for work. gcc, g++, npm, javac, etc. Maybe it'll make developing Android apps on an Android phone very viable. Java/Kotlin compiler

Maybe this may help make your old android phone age well like using a phone to be a Jellyfin media server or some home automation computer, voice assistant, rather than buy something from Amazon

I haven't tried it but there's a lot of computing power in modern phones that are wasted. Phones have been more powerful than Raspberry Pi's since forever and those are the backbones of so many things out in the world you'd generally never notice

Desktop Linux has a lot of familiar software that people use on Macs and Windows machines. Android getting access to desktop Linux applications makes it seriously usable as a dockable PC

You can do whatever you could do on a PC in a debian environment terminal. If you don't know what then you don't need it.