[jandrese]

DOS did it because CP/M did it.

If I had to guess I'd say CP/M was developed for extra dumb teletypes that needed both to properly handle a newline. So many quirks in terminals date back to the days when everybody was just making it up as they went along. Legacy support is the root of most braindamage.

[Someone]

Teletypes needed it for timing. https://en.wikipedia.org/wiki/Newline#History:

”The separation of newline into two functions concealed the fact that the print head could not return from the far right to the beginning of the next line in one-character time. That is why the sequence was always sent with the CR first. A character printed after a CR would often print as a smudge, on-the-fly in the middle of the page, while it was still moving the carriage back to the first position.”