A database is a collection of connected items (spreadsheet) that, when associated, creates a single record (row) inside a group of multiple records (table).
Your license contains your name, gender, height, weight, eyes and hair color, address, date of birth, license issue and expiration date, license class.
Because it's possible to have multiple people with the same name, address, gender, and description (like a father and son), driver's licenses all have license numbers.
Other types of relationships include One-to-One and Many-to-Many.
The most obvious is so you don't repeat the same data on every spreadsheet in your workbook.
For example, it's redundant, time-consuming, and resource-intensive to retype all the driver's names and birth dates from the Master database to the multiple Slave databases.