Here's a few ways you can use customer records in ShipStation:
Reporting: Customer records are used to create the information graphs found in the Customer Overview section of the Insights tab. You can also create a Customer Raw Data Export file from the Reports section of the insights tab.
Order Management: If you view a customer record from the Customers tab, you'll be able to see the customer's order history and manage their orders.
Quick Manual Shipment creation: You can create a shipment directly from a customer record, without having to create an order first.
Automation: Customers records can be tagged, which can help you filter orders and be used as criteria to trigger Automation Rules.