What is a firewall and do you need one?
A firewall controls which network traffic may reach your computer and which programs may phone out. You already have two: one in your router and one in Windows or macOS. Suite firewalls add easier controls and alerts on top; for most people the built-ins, left on, are enough.
Think of a firewall as the doorman for network traffic. Inbound, it blocks connection attempts from the internet to your machine; outbound, it can stop a suspicious program from sending data home. Your router performs the first job for the whole house simply by how home networks work, and Windows Firewall plus the macOS firewall handle the device itself, enabled by default.
So why do suites advertise firewalls? Control and clarity: the firewalls in Bitdefender, Norton, and ESET make per-app rules understandable, warn when a new program wants network access, and adapt automatically between home and public Wi-Fi. For users who want visibility into what their machine talks to, that is real value; G Data and ESET cater especially to that crowd.
The practical advice: never turn the built-in firewall off, let a suite replace it only if you actually use the extra control, and remember the firewall's limit: it manages connections, not content. A phishing site you visit voluntarily walks straight past the doorman.