The Listener component handles the "remoting" layer that connects the web server module and daemon halves of the SP. For most purposes, you can ignore it as an internal detail that doesn't require your intervention.

This remoting layer is an inter-process communication mechanism, but is not optimized to be used between machines. It's not secured, though people tend to gloss over that kind of thing as being "behind the firewall, so who cares?" (hint, you should). But, also, it is accessed frequently, typically at least once per web request. The best analogy would be the AJP protocol connector used between Apache and Tomcat, but is generally even less efficient. Used over a domain socket or loopback connection, the speed is not a major factor, but across a network, performance will suffer.

The actual <Listener> element isn't really used, as that's an extension point. There are two plugins that are typically used, and they're directly configured by the <UnixListener> and <TCPListener> elements.

This can be omitted from the configuration, in which case the TCP (on Windows) or Unix (everywhere else) plugins will be used, with the default settings described on their respective topic pages.