Firewalls are a great way to “control” the environment. They do play major part in protecting any sized business and the home as well:
Firewalls are know for putting a barrier between the internet and your internal network. You can use applications to block specific known threats by subscribing to services that update your firewall against known threats.
You can isolate your less secure devices like IoT devices from your more sensitive devices like you phones, computers, and tablets.
You can watch your traffic. I try to take a glimpse of who is scanning the network. You would be amazed of how many times someone, something, tries to scan your network. Shodan for one, I see retailers sometimes ping my firewall from time to time. Maybe to see if I’m connected?
Monitor Traffic flow. One of the biggest jokes around is when internet companies try to sell you bandwidth you don’t need. Regardless of what many say, streaming doesn’t consume as much bandwidth as many think, and not as much as the internet companies tell you. Firewalls can show you in data and graphics how much of your available bandwidth you are actually using as any time..
Spam filtering. It does a great job of filtering to.
Firewalls may not exactly be a control depending on the context, but they are an important part of your network security.