Prevent data breaches: Since a penetration test is a benign way to simulate an attack on the network, you can learn whether and how you are exposed. It’s a fire drill to ensure you’re optimally prepared if there’s ever a real fire.
Like most people, you probably think that quality assurance for software is both sensible and necessary before you roll out software into production. It’s sensible not because you don’t trust the software developers to do a good job, but because it’s good business practice to ensure that the code works as expected. It verifies that your production systems are secure.