The top use cases for known data -- especially when first getting started -- are using known customer identifiers such as email or SHA-256 email for ad targeting and ad suppression.  Here are some specifics:

• Build audiences of best customers and export to Google Customer Match, Facebook Custom Audiences, LinkedIn, TikTok, and other addressable ad platforms.  From there, build lookalikes for ad targeting.
• Export all customers to addressable media platforms above for ad suppression from customer acquisition efforts.
• For more sophisticated users, Customer Match and similar products can be used for copy / creative targeting.  For example, target copy & creative around renewal or upsell for existing customers.

When using ads use cases, be sure to structure campaign nomenclature and/or query string parameters such as UTM to capture the value of this first party customer data.  Typically, these high level tactics add a ton of value to marketing campaigns.  They are also critical given the diminished effectiveness of 3rd party cookies.

Of course, Cloud Connect can do a lot more.  As maturity advances, users might export lists to their email marketing platform for targeted messaging.