Today, most of the company need to connect to internet. Firewall is used to protect you internal network from internet attack. Firewall is different from router, it provide more security control, monitoring and prevent attack feature.

For example, by using a firewall you can monitor traffic usage of different device, is there any data send out to internet, or any device is using email to send out file etc.

Secondary, firewall provide better performance than router, router is used to support small scale of device to access internet, more the devices connected, its performance will be drop or cause the network unstable.

Thirdly, firewall can create rule to block external traffic, such as some PnP traffic which enabled by default in most of the router, it will cause security risk in internal network. But in firewall same feature will be control by access rule, it can reduce the security risk in network and let the traffic under monitor.