I expect this is to gain patents/technology around making autonomous robots, so that Amazon can create a warehouse system similar to Ocado, where robots, rather than people, pick groceries.
And what makes you think Amazon doesn't do that yet? Kiva systems did what Ocado does now a DECADE go. Amazon acquired kiva systems (infact, Amazon robotics was born out of this acquisition) long long ago and I'm sure can do a lot more than Ocado does now.
https://www.youtube.com/watch?v=4DKrcpa8Z_E