This is odd, I could have sworn mine rang the bell when the carriage return returned to the start of the line, so it was more of a "swoooosh ding!", but I watched a video and you're right. Very odd.
The bell was rang (rung?) when you were around ten columns away from the right side of the paper, as a notification to the user to manually use the carriage return bar, or use the return key on the fancy electric typewriters.
> 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.