Three key concepts of security are integrity, confidentiality, and availability.
Network firewalls are the first line of defense, intended to keep intruders out. Once a system is compromised, all of its data can be accessed, and if a bad actor gains access to one system, they effectively have access to all of the systems. This touches on both system integrity and data confidentiality.
Security is also related to availability, or uptime. If your network is compromised, it may be inaccessible to those who need to use it, making it useless. If I'm working from an office in Chicago, and the company's main DC is in Atlanta, I need the network to remain available to access critical files.