Why and How to make a Requirements.txt

Using a Virtual Environment to Avoid Seeming like a Sadist

TL;DR: $ pip freeze > requirements.txt

Why not just write pretty code and push it to GitHub like a happy little clam, and not worry about making a requirements.txt? If my code runs on my computer, why should I give a care about my python environment? What even is a python environment? Perhaps a reticulated python’s terrarium?