You have a repetitive task which you want to automate with minimal effort. Naturally, you decide to whip up a quick Python script. With a few handy libraries you pip installed, you can write the logic in fewer than 100 lines of code. Everything goes well.
After some time you realize you can re-use functionality in your script to solve other problems. Your script evolves into multiple scripts, modules, and eventually a library. All the while, you continue to install new dependencies with pip and your system package manager. Everything goes well.
The sky is clear and the stars are bright. Some embers from your bonfire were blown into a patch of dry grass by the wind. It begins to smoulder.
Others are interested in using your library. You happily share your code. Unfortunately, they can't get it to work on their computer. They have to set up an environment like the one on your machine. But how can they do that? You don't remember. After days of trial and error, they manage to get something to run, but there are still obscure bugs which you can't reproduce on your machine.
Eventually you install a few innocuous packages in your own environment and everything breaks. You don't know what caused the breakage, and you don't know how to go back to a working environment. If you did not use a virtual environment: congratulations, you now bricked your system's Python installation. Critical system packages don't work anymore.