Many source packages are not the same as what is on the source repository at
a given revision/tag.
For example, the configure script is usually pre-generated on the tarball
and not present on the repository.
Many other generated content is not present on the repository. One usual
case is the repository to have SVG files, but not the target PNG files used
by the application. The user could generate those but it would probably
require other software not needed for simple program compilation.