Why does ASP.NET perform validation on both the client and the server?

Client-side validation helps avoid round-trips to the server. Validating on the client ensures that the data is valid before it is submitted, in most cases. However, because validation might be turned off (or maliciously hacked) on the client, data must be revalidated on the server side. This provides full assurance that the data is valid while avoiding as many round-trips as possible.