Which --delete* to Use in 'rsync'

[Kenneth Jacker <khj@cs.appstate.edu>]

>From rsync(1):

     --delete                delete extraneous files from dest dirs
     --delete-before         receiver deletes before transfer (default)

     --delete-during         receiver deletes during xfer, not before
     --delete-after          receiver deletes after transfer, not before


I've always just used the first option (--delete).

Can anyone suggest when I might use one of the last two?

Thanks!
-- 
Prof Kenneth H Jacker       khj@cs.appstate.edu
Computer Science Dept       www.cs.appstate.edu/~khj
Appalachian State Univ
Boone, NC  28608  USA