1. Integrate Apple Pay into Didi Chuxing apps, which is estimated to have 300MM users in China[1]. Didi currently only accepts payment with Weixin(aka WeChat) Pay and Ali Pay (aka Ant Financial Services Group, owned by Alibaba Group).
2. Massive data points for developing Apple's self-driving technology. Didi operates in 400 Chinese cities with over 11 million rides per day, and accounts for 80% private car hailing market and 99% taxi hailing market.[2]
3. And yes, investing into Chinese tech sector give them a better leverage in negotiations with the government and also like sbuccini said, association with other Didi's major investors.
[1]: http://www.theverge.com/2016/5/12/11669178/apple-invests-1-b...