Environment values can be used in any Action of ur script, 
those act as Global Variables

The variables declared in Action1, we cannot able to use 
them in Action2. So, using Environment key we can access 
any variable in any Actions. These variables are known as 
environment variables

Environment parameters are especially useful for 
localization testing, when you want to test an application 
where the user interface strings change, depending on the 
selected language. Environment parameters can be used for 
testing the same application on different browsers. You can 
also vary the input values for each language by selecting a 
different Data Table file each time you run the test.
You can also create environment output values, which 
retrieve values during the test run and output them to 
internal environment variable parameters for use in your 
test.