svn-buildpackage and other scripts around it has been created to do the following things:
Keep Debian package under revision control, which means storing different versions of files in a Subversion repository.
Allow easy walking back trough time using svn command.
Easy retrieval of past versions.
Keep track of upstream source versions and modified Debian versions.
Easy installation of new upstream versions, merging the Debian changes into it when needed (similar to the uupdate program).
Automated package building in clean environment, notifying about uncommited changes.
Create version tags when requested to do the final build and update changelog when needed.
allow co-work of multiple Debian developers on the same project.
Auto-configure the repository layout, making it easy to use by people without knowing much about Subversion usage (mostly you need only the add, rm and mv commands of svn).
Allow to store only the Debian specific changes in the repository and merge them into the upstream source in the build area (which nicely completes build systems like dpatch or dbs).
If wished, keep the upstream tarballs inside of the repository.