[MrJohz]

Anything that is "super important and critical" will need to be validated on the server no matter what. But by pre-validating things in the browser, the user can have a more intuitive, seamless experience. Is the username you're typing already taken? No need to press the submit button, then scroll around trying to find the correct error message, we can give you feedback while you type.

You are right that there are tradeoffs, and I agree that client resource usage is under-analyzed, but sensible use of client-side code can make things like forms, visualisations, tables, etc much easier to use.