It exposes core business logic and data allowing the integration with other systems, mainly mobile and frontend applications, using REST web services.
These requests are stateful if you are using version 1 of the web services. A cookie is generated for the cart which must be sent in the subsequent requests.
Version 2 doesn't use sessions. The ID of the cart is used instead.
The SAP Hybris documentation has a good description of the required requests: OCC Sample Flows
–Based on Hybris version 6.5 and 1905