To use a very poor analogy would be:
We need to write the address of an event into a letter before we send it so the person knows where to go before the event instead of giving everyone the address when the event starts.
(this doesn’t quite the point across for SSR but it breaks down the logic of how CSR works).
We can’t modify that address once we have written the letter and tossed it in the mail, nor can we give relative directions like “turn left when you see this giant rock, then an immediate right”.