Siesta is a framework for Swift that dramatically simplifies working with RESTful APIs. And like many things in Swift, it is natively built around asynchronous execution. It may fire any number of requests back, and they may complete in any order that is undefined.
But sometimes, you need to execute things in a specific order. Like when the result of one call will change subsequent calls. A classic example of this is an API where you might need to create a folder first, then upload files into the folder you created. So the folder creation needs to happen first, then the file uploads can happen after.