Most important question is a matter of two things. History and genetics. You preserve history for it's own sake. And you preserve biodiversity to take advantage

Most important question is a matter of two things. History and genetics. You preserve history for it's own sake. And you preserve biodiversity to take advantage of all sorts of traits for breeding. One might be particularly cold hardy, another resistant to blight, and so on. Or not. You preserve them for their own sake. And also because people just love this stuff.