Every row in your database needs a name. For years that name came from one line of SQL:

CREATE TABLE orders (
  id BIGINT PRIMARY KEY AUTO_INCREMENT,
  ...
);

The database hands out 1, 2, 3, 4. They are unique, they are sortable, they are small. It is a solved problem — right up until the moment you have two databases.

Then it stops working, quietly and catastrophically. Both databases happily hand out ID 1. Two different orders, same identifier. Your foreign keys now point at the wrong rows, and no error was raised anywhere.