why we use virtual environment in python code example

Example 1: virtual env create python

#------FOR LINUX/MAC---------#
#installing venv 
sudo apt-get install python3.6-venv
#creating virtual env
python3 -m venv env
#activating virtual env
source env/bin/activate


#-------FOR WINDOWS----------#
#installing venv
py -m pip install --user virtualenv
#creating virtual env
py -m venv env
#activating virtual env
.\env\Scripts\activate

Example 2: how virtual environment in python works

>>> import sys
>>> sys.prefix
'/System/Library/Frameworks/Python.framework/Versions/3.5'