Drupal blocks are typically meant to store content to be displayed across several pages. For that use case it makes sense to have a separate admin interface for blocks. But often there's a desire to include additional single-use content in a region.
For example, you may wish to show a menu on every page in a section, but add a PDF download link that varies across pages. You certainly don't want to create a block for every link on every page. Furthermore, views, panels, and other modules may be overkill for what will amount to a couple lines of code.[...]