This article explains what you need categories and streams for
You need categories for the printed weekly program and for the similar experiences that you will find in each experience in the guest perspective as soon as at least one other experience has the same category.
You can find a detailed explanation in this article: What are the categories in the experience for and why do I need them?
Streams are used to sort your experiences thematically. The streams are mandatory to show your experiences in the web app and to integrate them via our widget codes on your website, in your correspondence tool, etc.
You can find a detailed explanation of our streams in this article: How do the streams work and why are some streams invisible and yet "visible"?