You can run CSJS to determine the following:
Run a web experience based on conditions that relate to the page that the guest is visiting.
Enable SItecore Personalize to access browser data, including the Document Object Model (DOM) from the browser. This enables you to manipulate the structure, style, and content of the experiment, including where and how you want the offer or content to display in the browser.
Use SSJS when using CSJS. This can include a full history of the guest as well as a continuous stream of real-time session data. An internal service known as the Guest Context Service generates the guest attributes on the server-side.