Port Scan's don't only need to be the tool for hackers, they area a handy tool for a network or server administrator. Especially when debugging firewall issues. For example want to see which TCP ports www.google.com has open?
nmap -sT www.google.com
The output of the simple example above might look something like this:
Interesting ports on py-in-f104.google.com (18.104.22.168): (The 1657 ports scanned but not shown below are in state: filtered) PORT STATE SERVICE 80/tcp open http 179/tcp closed bgp 443/tcp open https Nmap run completed -- 1 IP address (1 host up) scanned in 31.619 seconds
That shows port 80, 179, and 443 open, so if your trying to ssh into google.com you won't get very far.