Also, it makes it easier to determine when a sentence ends that contains an abbreviation. (This refers to when most people would use periods at the end of abbreviations.) I still use double spacing even though standard HTML does not display the double spacing on web pages.

The following sentences are an example of what I was saying:

"I sent the N.D.A. to N.Y.  It arrived three days later."

The single space after the "A" in "N.D.A." indicates that the sentence has not ended even though there is a period.