Its not something you need to go through everyday, but if you run a lot of applications or application servers (web server, ftp server, etc..), you will face a problem telling you that you can not start APP X because Port Y is being used without the OS telling you what application is using it.

The easiest way to know which application is using what port is to use the command : netstat -a -b -o -n

The command above will tell list all the ports under the application using it.