> Why? To take just one of many examples, so that you can do the same validation client side and the server side. You have to do the validation on the server b

> Why? To take just one of many examples, so that you can do the same validation client side and the server side. You have to do the validation on the server because the client can't be trusted, but if you only do it on the server you have to do a full page load to validate any of the inputs, give feedback, or vary the form fields displayed. e.g. how many times have you filled out a long and complicated form, pressed submit, waited several seconds, and then get dropped back at the same form where you have to hunt for the error message, change the requested field and try again. And heaven help you if you got multiple fields wrong, where the data from one informs the validation of another. Client-side logic can make this process much lower friction.