When you have a lot of software installed on your system, it might get
difficult to trace exactly which configuration files you need and which
ones you don't need.
Now imagine the following situation: a package's maintainer can declare the
per-user configuration/cache files used by the program, so they are added
to the dpkg database along with other files belonging to the package.
It would then be trivial (or at least possible) to write a simple program
which scans the user's home directory, and notifies the user of
configuration/cache files which do not belong to any installed program.