when is the Destination address affinity persistence be used ?

'Destination' refers to the destination address of the original client packet. One example for use would be with a virtual server that balances traffic to a set of caching web proxy servers. See the example in K2492.

Also good answers here.

The destination affinity would be to pool members. If you were to have a pool of forward proxies for outgoing traffic, you would want to persist traffic going to specific ultimate origin servers to tend to go through the same proxy (pool member), this would optimize cache hit rates.