The idea of the Static HTML Viewer module, is to bind a static HTML web site directly into Drupal.
This is not about importing a static HTML website into Drupal (this is already well documented). Instead, we are directly binding a website into Drupal.
This is very useful if you want to expose a set of HTML pages into your website without having to create a node for each page.
For instance, if you are a developer and generate documentation for your code (using Doxygen in PHP or Javadoc in Java), you will end up with a lot of HTML pages. You might want to expose those pages inside your Drupal website, in order to benefit from all the benefits of a Drupal install (themes, etc...) This module will help you do this: the content of every HTML page is copied in the content of the theme.
The closest module to "Static HTML Viewer" is the Import HTML module.
While the Import HTML module creates a new Drupal node for each HTML page it imports, the "Static HTML Viewer" does not create any nodes. Instead, it loads HTML pages directly from the disk and embeds them in the Drupal theme.