When I finish a sentence, I add one space and move on. Yet I find many people who enjoy adding two spaces after a period. Why do they do this?
The short answer is typewriters (the long answer being a bad habit). Double spacing comes from old manual typewriters—now a couple generations out of fashion—that employed monospaced fonts. Unlike modern fonts that proportion letter spacing per the width of each letter, monospaced fonts gave each letter the same width on the page, be it an i or an m. Adding a second space after a period in a monospace setting makes the end of a sentence more noticeable. Of course, that was over forty years ago. We only use monospace fonts today to emulate the "cool" look of typewritten text (minus the slow, noisy, and soemtimes clogged process that comes with writing on a real typewriter). So why do people, especially my young fellow millenials, still carry on this habit? Whether they want to act older than their age or never received the memo that double spaces are unnecessary (and strongly discouraged by modern typographers), I cannot say. Plus, most people who use double spaces do so inconsistently, forgetting a second space here or there.
If you are one of these double-spacers and have any lingering doubt, let me point out that modern typographers, MLA, and Chicago Manual of Style encourage one space. But even if a style guides okays the second space (APA), this issue falls in the domain of typographers, not stylists. And when Robert Bringhurst, author of The Elements of Typographic Style, suggests that you "use a single word space between sentences," then you better search and replace every doubled space in your writing here on out.