Information Security Managers are required to have a wide knowledge of information risk management, this means that the information security managers understand the security methodology behind the most systems. This knowledge allows the information security manager to take appropriate steps to secure the environment he or she is supposed to manage.
The information security manager is also supposed to have knowledge on topics like information security governance, information security incident management, information risk management, information risk compliancy, information security program development and management.
The best part on having an information security manager is the fact that they will be able to do threat assessment, checks on vulnerabilities and investigate the impact if something would happen to your company processes and workflows.
Of course I do not know your environment, it is impossible for me to pinpoint why you would need an information security manager in your company, but what I can do, is provide you the benefits of having an information security manager in your company.