Strongarm provides an easy way to override the default variables in Drupal core and contributed modules.
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 an opportunity to set the site_frontpage to whatever is best for their site.
One of the weaknesses of Drupal (at least up to and including version 7) is that content and configuration are mixed together in the database. This can make it difficult to migrate complex changes from one site to another. Many of these migration problems are partially resolved by Strongarm combinin Strongarm with Features,.