Your network is always changing. You are gaining new users and new devices. Those users and devices are accessing new applications and new services. And applications and devices that once accounted for a high percentage of network traffic may become far less popular over time.
All those changes may mean that you need new firewall rules or that you can delete some firewall rules that are no longer necessary.
It’s human nature to delay fixing something until it becomes critically important. But your firewalls are far too important for a reactive approach. You don’t want to be updating your firewall rules under pressure because you have suffered a breach or because users are complaining that the network is too slow. It’s far better to set up a regular maintenance schedule — perhaps quarterly or at least annually — so that you can make changes proactively.