Interesting. http://linux.die.net/man/2/sync says

    On Linux, sync is guaranteed only to schedule the dirty blocks for
    According to the standard specification (e.g., POSIX.1-2001), sync()
    schedules the writes, but may return before the actual writing is done.
    
    However, since version 1.3.20 Linux does actually wait. (This still
    does not guarantee data integrity: modern disks have large caches.)

So it seems like the sleep(2) is there to give the disk enough time to write the cache data.