as you said we can validate using server side validation but we can validate the user form using java script also.each time for validating user input if the request will go to server then it may increase network traffic.
Just to make it clear: There's nothing at all wrong with doing client-side validation. You're right about the fact that it does save server round-trips. What I believe the previous posters are trying to tell you is that you can't rely on client-side validation only . You must do the validation again on the server-side as an added precaution.