Why Normally HNSC is using the port 80 with blank host header setting for web application ?

Copy from : http://www.jeremytaylor.net/2013/03/04/host-named-site-collections-sharepoint-2013/

Note – HNSC can be created on a non default Web Application but you’ll need to add bindings in IIS for your HNSC as the Web Application isn’t listening to all port 80 requests and would need to know about the HNSC it hosts. 

Thats why a lot of blogs usually specify HNSCs on default web apps, because adding IIS bindings can get messy. 

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If you using the port 80 with empty host header then you are no need do manually do IIS binding for your site collection HNSC . else you need to do IIS binding for each site collection created by HNSC.