To have the reciprocal benefits on AA be similar to NW: pre-boarding for MVP's, upgrade certificates, and use of AA lounges as an AS Boardroom member when flying on AA. With the new AA extra legroom in coach, and the fact it's just about impossible to upgrade with certs on NW, I've been flying AA more often on transcons.
To be able to use AS web check-in when upgraded to F.
CS