The macro was made to help installing broken packages that did not use
DESTDIR correctly by overriding individual path variables (prefix,
sysconfdir, ...). Newer rpm provides fixed make_install macro that calls
make install with just the correct DESTDIR, however it is not available
everywhere (e.g., RHEL 5 does not have it). On the other hand the
make_install macro is simple and straightforward enough for us to use
its expansion directly.