I prefer my students to use two spaces for several reasons. First, since I can’t always count on them to use a period at the end of the sentence, nor to capitalize the first letter of the next sentence, sometimes two spaces is the only signal I get of a sentence break. Second, I find two spaces helps with the “thought break” between sentences. Third, it gives me more space to squeeze in proofreading symbols, since some of their writing is so bad the space between every double spaced line is full. Finally, I find it assists in the reader’s comprehension and when read aloud, the reader pauses longer between sentences, assisting in the audience’s comprehension.
But this is a style choice, not a mandate. Myself, since I learned to type that way, it’s reflex now.