Academia

I remember taking typing (on manual typewriters!) in high school before the personal computer.

I don't know if it was then, or 20 years later when I was taking a desktop publishing course, but I remember being told that, just like dashes, spaces come in 2 widths. The en-space, which goes between words, and the em-space, which goes between sentences. (There are other widths used in kerning, the spacing between letters.) The two-space convention of typing was an approximation of that.

This part I know I got from that desktop publishing course: As a result, the two space convention should be followed only when working with monospaced fonts, proportional fonts that don't offer an em-space, or software that isn't smart enough to use the correct space in the appropriate places and you're too lazy to do it by hand. (I used find and replace for that last case when em-spaces were available.)

Now, I can't say I really care. :)