Structure app provides APIs that allow you to access structures, integrate your add-on with Structure and extend Structure functionality. 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 app, or plan to create one, and you'd like to use the issue hierarchy provided by Structure – see Accessing Structure from JIRA Plugin.
You'd like to extend Structure, adding functionality to the plugin itself – read documentation about extending Structure functionality with additional plugins.
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.