Screens can be redesigned in days. The structure of a system's data shapes everything it can do, and changing it after the fact is slow and risky. That's why, on every system we build, the data model is designed and agreed before the interface.
A data model is the structure that describes what information a system stores and how those pieces relate — customers and their accounts, orders and their items, products and their locations. In a relational database, it's expressed as tables and the relationships between them.
A good data model reflects how the business actually works: one customer with several sites, one order with several deliveries, one product stocked in several places.
Moving data from spreadsheets or old systems is often the riskiest part of a project:
Backups, access control, encrypted connections and least-privilege permissions for integrations.