using the same download directory multiple times

After reading the comments in the local.conf file about the download directory (DL_DIR), I experimented with creating it outside of the normal <build dir> directory used by "source oe-init-build-env <build dir>" command. After completing a build successfully, I examined the download directory and I'm trying to figure out what is in there besides the tarballs that were downloaded and the .done files for the matching tarball. There also appear to be .done files that are not related to any tarball in the download directory.

What is the purpose of these files?

Apparently, all this works because I've recreated the same BSP in two different build directories with a common DL_DIR outside the build directory successfully.

Jim A