You're curious about what a multisite is? Multisite is a DNN variant that allows users to access several sites from a single DNN installation. It enables a network of areas to be run under a unified DNN interface. You can handle anything, including the number of pages, functionality, themes, and functions of the user. It's capable of managing hundreds, thousands, even (possibly) millions of pages.
Multisite is best used for building a network of sites that share similar features. It performs best when used on a small range of themes exclusively curated for network purposes, rather than enabling network users to add their unique pieces.
While controlling/managing several DNN sites or portals, you're not going to have to switch between systems, and it saves you a lot of time. Besides, you can build content once and then share it through the Site Community pages. Visitors of the web would find it easy, and they will use the same details to sign in to each website.