PATH is an environment variable in Windows. It basically tells the commandline what folders to look in when attempting to find a file. If you didn't add Python to PATH then you would call it from the commandline like this:

C:/Python27/Python some_python_script.py

Whereas if you add it to PATH, you can do this:

python some_python_script.py

Which is shorter and neater. It works because the command line will look through all the PATH folders for python and find it in the folder that the Python installer has added there.

The reason it's unticked by default is partly because if you're installing multiple versions of Python, you probably want to be able to control which one your commandline will open by default, which is harder to do if both versions are being added to your PATH.

It is not required to add every version of python to the path environment. You do need to add the directory containing the Python executable to the path if you want to run python or its associated tools (e.g. pip).

Here are a few approaches:

1. Add Python to path: There is an option to add python to the path during the installation. Make sure to check that.
2. Manually add Python to PATH: You can do it manually if you installed python without adding it to path. On Windows, you can go to System Properties > Advanced > Environment Variables, and then add the path of the Python executable (e.g. C:\Python39) to the path variable. On Unix-based systems (Linux or macOS), you can modify your shell configuration file.
3. Use a virtual environment: Instead of relying on system-wide Python installations, you can use virtual environments for your projects. This way, each project can have its own isolated Python environment. Use tools like virtualenv (for Python 2) or venv (for Python 3) to create and manage virtual environments. Activating a virtual environment automatically adjusts the path for the current shell session, so you don't need to worry about varying versions.

Regarding updating to newer versions of python: The path needs to be updated manually or reinstalled by setting the path while installing it.

Regarding updating your shell: As long as the path modification is in a shell configuration file (e.g:.bashrc), it should persist across its updates.