Strongarm gives site builders a way to override the default variable values that Drupal core and contributed modules ship with.
It is not an end user tool, but a developer and site builder tool which provides an API and a limited UI.
An example of such a variable is site_frontpage. In Drupal this defaults to node, which ensures that the front page gets content as soon as some exists, but for many Drupal sites this setting is simply wrong. Strongarm gives the site builder a place in the equation - an opportunity to set the site_frontpage to something that makes sense for their site.