Every large paper needs an introductory piece that tells the reader what to expect. This introductory piece gives the reader an overview and tells the reader what to expect. But because its purpose is to give the reader an overview and to tell the reader what to expect, it also gives you, the writer, an overview and tells you what to expect. It is a sort of plan for the rest of the work.
The first draft of an intro can be written much more quickly than the first draft of any of the later chapters, so it's a small task, and therefore a good place to start. It's just a draft to be moved through quickly to help organize your thoughts. That organization then informs the first draft of each of the chapters that follow. It's true that in writing the first draft of the later chapters you will see the arguments in greater detail and depth, and that that knowledge could help you write the introduction, but it could just as well help you re-write the introduction.
But for this schema to make sense, you don't want to spend too much time on the first draft of the introduction, you want to move through it quickly and use it to help you plan the rest of your writing.