ipsec pass-throgh and NAT-T two different things - same family but diferent purposes, separate them to not get confused.
Ipsec-pass-through beside (inspection engine - which is another topic) it opens up Ipsec VPN ports, in earlier PIX versions 6.x or bellow you had to open up specific ipsec ports by access list instead so that your inside users could vpn outbound to other vpn gateways. In ASA 7.x above you no longer need to do acls to accomplish this, inspect ipsec-pass-through does it.
http://www.cisco.com/en/US/docs/security/asa/asa80/command/reference/i2.html#wp1721168
As for enabling nat-t, short answer simply allows the PIX/ASA or IOS UDP 4500 the detection of NAT devices between them and allow futher negotiating UDP encap ipsec packets.
Coun't explain it better here - to get the picture read this entire link - See number 4 ( the problem ) http://www.ittc.ku.edu/~kpm/ipsec_udp_encap/
Couple of other references - Read the RFC3947 and RFC3948
http://www.unix-ag.uni-kl.de/~massar/vpnc/docs/rfc3947.txt
http://www.unix-ag.uni-kl.de/~massar/vpnc/docs/rfc3948.txt
Jorge Rodriguez