Do you remember dot matrix printers? These early generation printers were affordable for home use, but widely spaced pixels made their output tough to read. When run on the draft setting, dot matrix printers were intoxicatingly zippy. To print a document with a more humane, higher density output, however, the term-paper writer was forced to watch the ink head take three passes at each line of type—a steep (and excruciating) investment in time.
As I progressed through my college career, more and more professors declared that they would no longer accept work produced on dot matrix printers. In my heart of hearts, I could not blame them. I can only imagine what a weekend of slogging through hundreds of pages of weakly printed copy would have done to the eyes of those professors.
The new edition of the Publication Manual recommends that authors include two spaces after each period in draft manuscripts. For many readers, especially those tasked with reading stacks of term papers or reviewing manuscripts submitted for publication, this new recommendation will help ease their reading by breaking up the text into manageable, more easily recognizable chunks.
Although the usual convention for published works remains one space after each period, and indeed the decision regarding whether to include one space or two rests, in the end, with the publication designer, APA thinks the added space makes sense for draft manuscripts in light of those manuscript readers who might benefit from a brief but refreshing pause.
As I learned in college, it is never a bad idea to consider the eyes of the person reviewing your work.