[pf4freebsd] why multiple CARP groups

[sam]

Hi,

I need to get adviced by someone  for the usage of CARP+pfsync.
With the BIG example as described in the following page:
http://www.countersiege.com/doc/pfsync-carp/#big
I don't understand why create a different CARP group for each 
application server instead of using only one CARP interface for 4 
internal application servers is better.

With only one CARP address for 4 application servers, traffic still can 
be redirected to another app server if one is died. Unless one CARP 
address is not efficient.

Can anyone please explain the difference using multiple CARP groups 
instead of one CARP address?

Thanks
sam