I think what Google is doing here is using AI to make its products smarter and then feed that data back into AI to make AI smarter. That's Google's end goal. To use AI to make its product better and frictionless.
Minus "AI" and it's like the same goal for every product/service. I get AI but the term is so over-used now it's just implied as "smarter" which is subjectively true. I don't always need smarter products, rather more relevant and useful. The simple fact that a poor UX in front of "smarter" services can destroy the whole concept.
I do think there is a bigger picture than reservations so for example if I knew that an action just took place the opportunities for highly targeted advertising knowing the context of what just happened could be significant. Feed Google's cash cow.
Google has dropped hints on how it may use Duplex in the way you say - reservations are just a way to train it. An example is to get better data on businesses in Google Maps by calling up businesses to see if they are open or closed on certain days, thus helping to improve one of their key platforms.
