Apparently, two spaces after a period was used in the typewriter days to help readability. Kerning (especially on web pages) helps obviate the need for this to

Apparently, two spaces after a period was used in the typewriter days to help readability. Kerning (especially on web pages) helps obviate the need for this today. However, there are still people who advocate its use. I do it out of habit, but can probably turn it off now that I think of it.