When download python from https://www.python.org/ and install it, I saw 2 apps installed, it is easy to see the purpose of IDLE but I am confused with the purpose of Python Launcher. What does Python Launcher do ? When do I need to it?
For running Python on a mac start at Python's documentation
Python Launcher allows you to run python scripts from the desktop.
The documentation says
To run your script from the Finder you have two options:
Drag it to PythonLauncher
Select PythonLauncher as the default application to open your script (or any .py script) through the finder Info window and double-click it. PythonLauncher has various preferences to control how your script is launched. Option-dragging allows you to change these for one invocation, or use its Preferences menu to change things globally.
From other Stack Exchange questions I think Python Launcher is not part of the python that Apple provides as part of the OS. But as you really should not use that python I have never tried to see.