It can be used for two purpose
First, those without write access to the BLFS SVN server can use it to generate patches to send to the BLFS-Dev mailing list. To do this, simply edit the files in your local sand box then run svn diff > FILE.patch from the root of your BLFS directory. You can then attach this file to a message to the BLFS-Dev mailing list where someone with editing rights can pick it up and apply it to the book.
The second use is to find out what has changed between two revisions using: svn diff -r revision1:revision2 FILENAME.