I’m not sure of the “actual” answer, but my guess is that they, in part, help us to break up words and phrases, making them easier to read. Think about Latin during the Roman Empire: no spaces between words and little to no punctuation. ifyoureadlatinyoumightbeexpectedtoascertainthemeaningofwholesentenceslikethis. It’s very difficult to read, our eyes just gum it all together. So spacing and size changes allow our eyes to more easily identify words. that and it helps us know who’s an a-hole on the internet.