what is the advantage of using rsync over sftp

[Tapas Mishra]

What is the advantage of using rsync over scp or sftp?
On same list I have put up a problem of taking backup of an LVM.
Which I mentioned here
http://forum.nginx.org/read.php?26,135008,135251#msg-135251
I used rsync but I am not clear with what is the advantage of doing
all this hard work.
What I saw was a snapshot lvm was created which was mounted some where
and  I copied
complete directory structure to some remote location.
What is the advantage of all this hard work can simple scp or sftp not
do the same thing?

-- 
Tapas