PIX required NAT to move between interfaces. This was required even if the IP addresses didn't change. The PIX became the ASA, and the ASA continued using globals, statics, and conduits. In 8.0 no NAT control was introduced eliminating the requirement to NAT everything. I hope that helps.
Back in those days, with PIX/ASA, traffic flow is controlled by the security-levels or ACL's (ACL's override security levels). NAT-control came as an additional mechanism to control traffic lows through the ASA: assuming the ACL would allow it, a NAT statement had to be matched as well, in order for traffic to be allowed.