What is setup.py for in Python?

Question

Python

Good afternoon! I am learning Flask from various manuals and books. But here I decided to raise an example from the documentation.
I cloned the repository and found setup.py in the root of the project. What is it for? Naturally, I started googling.
I came across this article about distributing packages , read about setup tools and still didn't get it.
Rather, I understood, but not quite. Let's say I'm developing a library with functions. Then everything is clear - this is necessary so that it can be installed, for example, in a virtual environment and used in other projects.
But when it comes to a standalone web application - what is all this for? Why is it not enough just requirements.txt to install the necessary packages, but for commands from the terminal the same click, For example?
What is setup.py for in a separate, standalone project?

Answer 1

read pip how packages are distributed
section Argument Handling