The double spacing is also (mainly?) because the formatting dates from an era of manuscripts typed on a manual typewriter, with a single choice of fixed-width (

The double spacing is also (mainly?) because the formatting dates from an era of manuscripts typed on a manual typewriter, with a single choice of fixed-width (large) font, and a fairly limited ability to make generous margins. In such a context having blank lines is the easiest way to give proofreaders a place to write comments and corrections. The typewriter legacy is also how we get two spaces for new sentences, and outrageously wide half-inch paragraph indentation, with indentation even for the first paragraph of new sections. It’s in my experience just as easy to write notes and corrections on documents which are written in smaller type with standard amounts of leading in a reasonable-width text block, with generous margins. And it’s much easier to read such documents. Unfortunately, since the typewriter format was standard at the time when computers started to be used for manuscripts (and school assignments, and theses, and legal documents, etc.), it just got copied over for computer-prepared documents, so we ended up with a standard of a 7.5" wide single column of 12 point type, double spaced. Microsoft enshrined the 12 point size, 1 inch margins, half-inch tab stops, Times New Roman typeface, etc. as defaults in Word, and now all kinds of document specifications are stuck limping along with forced ugly semi-legible formatting.