DonaldKnuth designed and implemented it because he was dissatisfied with the then current state of the art in computer book layout (they used to look like printed out textfiles, complete with ASCII art for figures). The language implementation's version number converges asymptotically towards pi, and is only updated to fix bugs. Since version 3, there have been exactly 5 bugs so the version is currently up to 3.14159.
Lesser mortals use LaTeX, a set of wrappers around it.
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