Why is it not possible to set the headers after they are sent to the client?

Question

Node.js

I'm having a problem redirecting the user to another page after a while:

return(setTimeout(function restart(){
                res.render('index')
            }, 4000));

The problem is in this piece of code (
the error sounds like this:
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (_http_outgoing.js:561:11)

Answer 1

Why can't you fry potatoes after you've eaten them?

Answer 2

How does the HTTP protocol work, you certainly do not know?
In short:
This is a text format.
Headers come first
Then double line
feeds Then page data

Заголовок1=значение
Заголовок2=значение
Заголовок3=значение

Тело страницы ....

If you send data, then where to stick the headers?