Calibri was a big deal when introduced because, along with the other fonts in that cohort[0] (like Consolas), it was optimized for subpixel rendering (known on Windows as ClearType), which was a new thing too, and made huge difference in quality and comfort of reading text on flat panel displays. It was a good, practical reason to create new set of default fonts.
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[0] - See: https://en.wikipedia.org/wiki/ClearType#ClearType_Font_Colle...