there is 2 types of natting.
client nat and server nat.
You will normally do server nat unless all your servers share 1 loopback ip address.
In this case, you use this loopback ip address as the vserver address and you don't do nat.
Otherwise you need server nat so the ip address of the server is used when forwarding the traffic.
Client is used when you want to hide the client ip or when you want to make sure the response from the server will be sent to the CSM.
Most of the poeple do not use client nat.
Gilles.