A column called `status TINYINT NOT NULL` in a table you've never seen. Is `1` active? Pending? Enabled? Is `0` deleted or just inactive? The column type doesn't tell you. Neither does the column name. The fix is one line of DDL nobody writes.
An `INSERT INTO order_items (order_id) VALUES (9999)` against an order_id that doesn't exist. With a foreign key the database rejects the row. Without one, the row lands and the corruption surfaces months later when finance can't reconcile a report or backups have already rotated past the window.