Store one fact in one dependable place
Suppose a shop puts a customer’s current phone number on every order. If it changes, some orders may still show the old number. A normalised database keeps the current number in one customer record. Each order stores a customer ID to link to that record; this ID is a foreign key.
An order can still keep the delivery address used at the time, because that past detail should not change when the customer moves.
When copies are useful
Teams sometimes copy selected details on purpose to make reports or screens faster. This is denormalisation. They then need to keep those copies consistent when the current detail changes.