Verifying understanding of backup-dir vs compare-dest

[Suresh Govindachar]

Hello,

  Say one starts with creating an archive 

       rsync work -> archive

  and periodically (below, i = 1 to N) does 
      
       rsync --backup-dir=a_<i>      work -> archive
  and  rsync --compare-dest=archive  work -> b_<i>

  Then suppose one wants to recover the work directory as it was at
  time k.

  Using the b_<i> directories, one would merely merge b_k with the
  archive directory.

  But using the a_<i> directories one would have to merge a_<j> with
  archive starting with j=N and going down sequentially to j=k.

  Is my understanding correct?  More importantly, I would like to
  know under what conditions and for what goals people use
  --backup-dir;  and likewise when and why do people use
  --compare-dest?
  
  Thanks,
  
  --Suresh