Why does the server verify that the form is empty and the client still needs to verify it?

Question

I see that the server has verified that the user's account and password are empty. Why does the client's js have to verify whether the form is empty? I don’t understand!

Answer 1

Double verification is required for security. On the one hand, js verification is to provide a better user experience. On the other hand, if the verification fails, you can skip the procedure and save server-side expenses.

Another program verification is a must. Yes, I can directly use a program to simulate your form submission without going to your front-end page. Do you understand?