Somewhere in the miasma of bandwidth-wasting childish prattle, someone might have answered this, but I'll go anyway.
Back in the day when print press type was set by hand, it was difficult to keep track of certain things--partly because the type was set backward. It was, for instance, difficult to distinguish between "p" an "q" when viewed backward, hence the phrase, Mind your p's and q's.
Likewise, it was difficult to discern the beginning and end of sentences, further compounded by limited font availability. Editors, too, who spent/spend all day reading/correcting manuscripts found them difficult to read after a while, so easy-to-read manuscripts received preferential treatment. Thus, doubles-spaced sentences and paragraphs became the standard.
Today, with desktop publishing and automatic text justification, extra spaces are unnecessary and actually counter productive. So, if banging out something on a typewriter, two spaces. In a word processor, one space.
(Full disclosure: I am the editor of a print magazine as well as a book author.)