It's not a "type of website", but instead "location of website", identifying the network that the website lives on. Let's say there's an intranet, internet and extranet available. Identifying the specific network that a resource lives on is important to distinguish if it's publicly accessible and what handling procedures are used when working with it as dictated by the policies of the particular network it resides on.

This is a bigger deal in the government, especially the DoD, where there might be an Internet website, a NIPRnet website, a SIPRnet website, a JWICS website, etc. So, sometimes it's appropriate - for most companies, probably redundant.