It's not the rarest. That honor goes to Newton's original manuscript, which the Royal Society (Newton's academic home for much of his life) is likely to keep for as long as possible. Another copy bound in similar material went for a relatively modest $2.5 million back in 2013.

To call Principia Mathematica important would be an understatement. It's the book where Newton sets out his laws of motion, including gravity. Many of the concepts outlined in the title would be cornerstones of physics for centuries (including today), and wouldn't be significantly questioned until the 20th century through relativity and quantum physics. They formed the basis of industrial breakthroughs, and helped explain or refine key astronomy concepts that we now take for granted: the elliptical orbits of planets, tidal effects and even the not-quite-spherical shape of the Earth. The very fact that you can read this article partly explains why Principia fetched a high price -- the technology you're using might not exist if it weren't for Newton's findings.