Comment by mtpn

The anecdote I heard was that the extra space made it more clear when sentences were ending based on how the fixed-width typeface would read. With proportional fonts the spacing is already clear enough with 1 space, and that has become the conventional thing. So adding the extra space creates too much space now. It certainly sends a signal to me that the person has absorbed a “rule”, but not its purpose, which I think doesn’t reflect well on that person. But on the whole that’s a minor side note and I try to not go overboard interpreting details like that.