HDFS-2289 downloads commons-daemon TAR in the hadoop-hdfs/target/, earlier patches for HDFS-2289 were using hadoop-hdfs/download/ as the location for the download.
The motivation not to use the 'target/' directory is that on every clean build the TAR will be downloaded from Apache archives. Using a 'download' directory this happens once per workspace.
The patch was also adding the 'download/' directory to the .gitignore file (it should also be svn ignored).
Besides downloading it only once, it allows to do a clean build in disconnected mode.
IMO, the later is a nice developer capability.