Must there be a firewall between companies? Why?

It is necessary to maintain enterprise security. There must be a firewall between companies. From the enterprise's point of view, firewall is a standard for enterprises to maintain enterprise security. Enterprise networks usually involve multiple departments and users, many of whom need to use sensitive information and data on the network. These threats may lead to data leakage, system paralysis or corporate image damage, so it is very important to use a firewall.