Nobody sets out to learn how to write 800 words that are exactly 800 words. You learn it because a page has a hole in it and the hole is a fixed size.
What looks like a constraint turns out to be an editing engine. A fixed length forces you to decide what the piece is actually about within the first hour, because you cannot afford to find out at the end. It kills the second-best example. It removes the paragraph you were fond of.
I still write everything to a target, including chapters nobody has asked to be any particular length. The number is arbitrary. The discipline is not.