Overpaying your mortgage can reduce interest and shorten your term, but it is not automatically the best use of spare cash. The right answer depends on your rate, emergency fund, other debts and flexibility needs.
Extra payments reduce the balance earlier, so future interest is charged on a lower amount. Over time that can save a substantial sum.
Compare your current mortgage path with one or two overpayment scenarios rather than assuming the answer.
Run your own scenario and compare one or two alternatives. That usually reveals the real decision much faster than generic advice.