He (cao cao) gain getting his ass whooped the second time (no im just joking..)
Seriously though i'd say the one thing Cao Cao gained out of the campaign was actually to see who would be holding most of the power after his death.The campaign really shows a transition away from the familiar generals of Xiahou Yuan, Cao hong, and Xiahou Dun(who was with the army) to the more importance placed on Zhang He , Guo Huai(who advised Cao Hong during the battles around LAngzhong)Xu Huang, who proved a competent duelist against Liu feng, but an unadaptable commander(as seen with wang Ping )and around Sima Yi, who advise the campaign to happen intially.Of course ZhangHe Xu Huang and Sima play a major role in the wei Kingdom after Cao Cao dies,(guo a backseat role to Sima Yi)and this is really the set of campaigns whereCao CAo and the readers and history realize that his old Wei guard simply won't carry into the era after CAo's death. Its like the transitional campaign where we see old tactics and armies and generals fade and new ones emerge(ZHuge Liang for shu)as well as the less importance of the duel as opposed to battle, after the Zhang He -Zhang Fei duel, the only major duels following in th SGYY are the Wang Shuang ones, Zhao Yun vs Han De and sons and of course the various Naman Campaign ones(Zhurong etc) and you begin to see less and less battles opening with a duel adn more and more relying on skill and strategy to win wars(im leaving out Jiang wei v Guo huai on purpose its just too far after this time frame).
Zhang he plays the main role in two or three of the Sima yi v Zhuge Liang Campaigns, Sima Yi becomes wei Cheif Commander and power holder, Xu Huang's inability to adapt gets him shot by Meng Da, and Guo Huai rises thru the ranks behind the scenes. My only q is where is Cao Zhen in all this?
"We Will Show Wu The Meaning of Fear!"-Cao Cao in DW6
"Politicians Are all the same all over, They Promise to build a bridge even when theres no river"-Nikita Khrushchev