Why is port scanning useful for hackers?

What will be an ideal response?

Port scanning helps you answer questions about open ports and services by enabling you to quickly scan thousands or even tens of thousands of IP addresses. Many port-scanning tools produce reports of their findings, and some give you best-guess assessments of which OS is running on a system. Most, if not all, scanning programs report open ports, closed ports, and filtered ports in a matter of seconds. When a Web server needs to communicate with applications or other computers, for example, port 80 is opened. An open port allows access to applications and can be vulnerable to an attack. A closed port does not allow entry or access to a service. For instance, if port 80 is closed on a Web server, users wouldn't be able to access Web sites. A port reported as filtered might indicate that a firewall is being used to allow specified traffic in or out of the network.