Many learner drivers go into the driving test without ever previously taking a mock test. Although it is not essential to take a mock driving test it can significantly improve your chances of passing the real exam.
A mock driving test allows you and your instructor to identify any areas of your driving that need improvement before taking the DSA practical driving test.
It is of course significantly cheaper if you fail a mock driving test than if you fail the real one. Failing the real DSA test will involve more test booking fees, more driving lessons and there is always a waiting list to take into account. (To take a mock test with your instructor is the standard lesson price)