What’s the point of port 80?

Web. The protocol or service generally associated with this is HTTP.

Transport Protocol

TCP

Safety precautions

Redirect to HTTPS when sensitive data or accounts are used.

Should we open this port in the firewall?

Only if a service is actually needed the use it. Apply the principle of any privilege: limit the authorized sources, destinations and interfaces, and then log connections when relevant.

How do I check this port?

Test-NetConnection SERVER -Port 80

Under Linux, you can use nc -vz SERVER 80.

Port/service associations are common conventions. An application can be configured on another port.