A firewall is the gatekeeper of your network. A good firewall, properly implemented, gives you not only protection from the outside world but also the ability to detect, analyze and visualize intrusion attempts and data flows in an audit-able way throughout your organization
It allows you and your employees to connect to your machines remotely in a secure manner.
It allows you to segregate your networks, and prioritize traffic eg Voip over others
Domain Controllers, Active Directories
Any network aside from an external firewall also needs an internal access control system. An Active directory controls login info for all employees, their read and write access to different information within the organization.
It allows us to control the movement of information, eg should people be able to copy company data onto usbs or not, and also better monitor and audit devices.