for what do the .env is used code example

Example 1: what is environment variable

Environment Variable , a variable that accessible
only when the environment is active 

Usually used for an app with multiple environment ,
we can use env variable to store variable with same name
and different value to store environment specific data 

for example : 

xxxNamed app has 3 environments with different
URL and Crendentials 

But all endpoints are exactly the same no matter what
environment you work on 

so we can create 3 environment called 
QA1 , QA2 , QA3 and run same set of request 
by selecting different environment.

Only one environment can be active at a time.

Example 2: what do you mean by environment variable

An environment variable is a dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs