Here's an example of what happens when line feeds are inserted manually.
The first screenshot looks OK, because the screen width is equal to or greater than the line width that the OP decided was appropriate.
The second shows what happens when the screen width is reduced - in this case by resizing the window, but would also be the case where the actual screen size is smaller - e.g. mobile or tablet. As you can see it makes a right mess and far from making it more intelligible, it has the opposite effect. I've left the quoted text in place because it hasn't been hard-formatted, and the line lengths are managed automatically and correctly by the software, so it remains readable.
70 characters per line may perhaps make sense in a printed book, but not where screen width is variable, where it is probably better to leave it to the greater intelligence of the software to manage the formatting. If anyone has difficulty reading and taking in content on long lines, they are at liberty to make their window narrower.
HTH
SD