Port 80 is the default port. It's what gets used when no port is specified. 8080 is Tomcat's default port so as not to interfere with any other web server that may be running.
If you are going to run Tomcat as your web server, the port can be changed to 80 so that visitors do not need to specify it.
If you are going to front Tomcat with IIS or Apache for some reason, that server will listen on port 80.
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