The default WooCommerce session table only stores the basics of a cart in session. CoCart provides additional data that maybe required to help you and other add-ons/extensions developed by CoCart or third-parties.
Such as when the cart was created. This information is only stored in the browser session.
Also the source of the cart it was last saved. For the web it will be
WooCommerce and for your headlesss store
CoCart. This lets you know which version of your store your customers are shopping from should you have both web and app versions.