Data Cloud allows Salesforce customers with either Enterprise or Ultimate(+) to get started with one of two use cases:
Sales: Consolidate your data across orgs and systems so you have a centralised source of truth with a standardised set of data. Use this information to better segment and target prospects for improved ROI%.
Service: Personalise the customer experience by understanding purchase history across multiple orgs and data sources. Whether this is via email, text, app messaging or an actual service professional speaking with a customer on the phone, you will be able to get a full view of the customer that unlocks the ability to provide the best experience.
In either case, you are using the ETL process (Extract, Transform, Load) to make data make sense within the CRM and, subsequently, Marketing Cloud!