Use Save to Disk to save your library as a backup according to whatever template you set in Preferences. Many people do this because they prefer some method of finding books in the filesystem instead of through calibre, and want a fine-tuned structure.
If you just need to transfer the library, copy your whole Calibre Library folder over to the new computer. See:
http://manual.calibre-ebook.com/faq....ter-to-another