Your statement that you had to use === makes no sense. Something else in your code must be wrong.

System.getGlobal 'Purpose' from DEV Guide

Retrieve a string value stored in the global variables list.

Return
String
The value of the global variable, or null if the name is not found.

In your case, either the value returned was null or maybe had some extra characters on it.

null == 'hello' => false
null === 'hello' => false

The double equal does type coercion and null and a string are not the same.

Sounds like the first time you did this, maybe the global was not set prior to your executing the script but sometime after, it was.