Server side validation requires a whole page round trip. The user will have to wait, lose their scroll position... you can't rely on client side validation only, but neither is it a gimmick.
Server-side validation relying on JavaScript. You still have to wait for the server to respond, and now you need to implement UI to tell the user something is happening in the background. But at least you can start proactively validating before the user actually submits the form, so there's that.
Server side validation requires a whole page round trip. The user will have to wait, lose their scroll position... you can't rely on client side validation only, but neither is it a gimmick.