[Someone]

> as a notification to the user to manually use the carriage return bar

It also, probably more so, was a signal to the user to start thinking about how to break the current line. You couldn’t type and, upon realizing the word you were typing didn’t fit the line, backspace and type a hyphen.

‘About ten columns’ then is a reasonable number. Of course, longer words exist and aren’t extremely rare, but those would have a reasonable hyphenation point that you could and would want to use.