Your "get out of jail free card" 

As you perform penetration testing, you will uncover multitudes of information that are valuable to real-world attackers, and you will also be performing activities that are illegal. The only thing that separates a penetration tester from a malicious hacker is permission.

Obtaining the relevant permission forms your "get out of jail free card". The permission that is provided by the business details any constraints and authorizes you to perform activities defined in your scoping agreement. 

It's a formal approval from the business to begin the penetration test.