Client-side validation checks are preferred to reduce the load on the server. Suppose there are 5 fields in the form and you decide to use server-side check. Every time there is an invalid input from the user, the server transaction is wasted because the input is sent to the server, gets validated but the data doesn't get stored because it is invalid. In such cases, using client-side checks reduce the requests to the server because only valid inputs are sent to the server. I think you were asked to use client-side checks because the input data is not dangerous to the functions of the web application.