Purpose is to store a link to an external URI such as the authors 's website (http://blah), or a folder/file of stuff you have for the author (file://blah), or other programs such as EverNote (evernote://blah) or OneNote (onenote://blah) that publish a URI for each unit of stored information (e.g. note), in which you maintain author specific info.
If present then that's where you'll go when you click on the author's name in the Book Details (right sidebar) panel. If it's absent then you'll go to whatever is specified in Preferences->Look and Feel->Book Details
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