Validating first on the client side gives immediate feedback to the user. Showing errors as the user works their way through a form.
Sometime customers think they know what they want, but the reality can be very different.
The worst way to design a website is to have five smart people in a room drinking lattes. The longer you leave them in the room the worse the design becomes. The next worst way is to have 15 customers in a room drinking lattes. What people say they do and what they actually do are rarely the same thing.