What is the use of CASCADE CONSTRAINTS?

Whenever rows in the master (referenced) table are deleted, the respective rows of the child (referencing) table with a matching foreign key column will get deleted as well. This is called a cascade delete.

Example Tables: Customer(customer_id, cname, caddress) and Order(customer_id, products, payment)

Customer is the master table and Order is the child table, where 'customer_id' is the foreign key in Order and represents the customer who placed the order. When a row of Customer is deleted, any Order row matching the deleted Customer's customer_id will also be deleted.