For larger software projects, there are often large features that will require several smaller tasks. This could also be accomplished by creating sub-tasks.
An example. Jira could use a discussion forum under tasks so that developers could communicate without using email, and be able to archive conversations about functionality for later. This new feature would require several things. The first step would be design, follwed by database structures, then interface, and finally testing/promotion.
Each step would be setup, however, step 2 or 3 could not be started or edited until step 1 was completed.
This feature is really about group tasks together and being able to effectively manager bigger tasks with small sub-tasks.