Full visibility across the mobile operation is an important aspect under GDPR. You can’t detect potential problems if you can’t see them. If you can't detect, you can't act fast enough or fail to act at all.
Having visibility helps you detect issues in case of security incidents. Knowing if and how much of your sensitive data has been breached helps you determine whether you can fix issues or have to report the incidents to authority.
Improving the security posture of your mobile app is only the beginning to meet GDPR requirements. Organizations also need to have sound incident response mechanisms in place to ensure quick and proper responses during security incidents.