Structure Plugin contains APIs that allow you to access the hierarchical lists of issues from other plugins and applications. Here are the typical use cases:
You customize JIRA for your customer or employer, and you need to integrate Structure with some other in-house system – see section about integrating plugins and Java API reference.
You have your own great JIRA plugin, or plan to create one, and you'd like to use the issue hierarchy provided by Structure – see Accessing Structure from Your Plugin.
You'd like to extend Structure, adding functionality to the plugin itself – read documentation about extending Structure functionality with additional plugins. (You can also extend the functionality of the Structure plugin itself.)
You need to get or change issue hierarchy remotely from some automated scripts or a client application – read about accessing structure data remotely and Structure REST API.