Apache Extras is aimed primarily those who are unable or unwilling to licence their code under the Apache License V2, but want to signal their relationship to one or more Apache project community.
One example of this is my own Drupal connector for Apache Wookie (Incubating). This needs to be GPL licensed due to the Drupal dependency, but it contains Apache Licensed code as well. Consequently it cannot be hosted at Drupal, nor can it be hosted at the ASF. Now, with Apache Extras, it has a home that is associated with at least one of those organisations.
A second group of projects that may choose to host on Apache Extras are those who wish to manage their projects in a way that is not aligned with our own collaborative consensus based processes.