Eugene
The purpose of this is
1) so that a host on the inside sending a packet to a machine on the dmz will appear on the DMZ as it's real IP ie. 192.168.0.x
2) so that a machine on the DMZ can send a packet to a host on the inside using the real IP addresses of the inside hosts ie. 192.168.0.x
Put more simply, to all intents and purposes it "turns off" NAT between inside hosts and the DMZ.
So why do you need to do it ? Because even when you don't want to NAT, ie. change the actual address from one IP to another, you still have to tell the pix you don't want to and this is how you do it.
Note that there is an option on v7.x code and upwards to turn off nat altogther ie.
no nat-control
Jon