parts

In contrast of several other compression formats (such as tar.* or zip), this format holds the Table of Contents of the archive in one block at the beginning. The files are also compressed individually. This produce a slightly larger archive size, but through network you are able to download only the table of content or only one (compressed) file.

In the practice this is handy in case of binary package update. The library (and the command line tool also) has interface that downloads only the binary difference (per file) of a directory.