This is allowing a simpler flow to know whether playback started or whether it failed. It also allows the UA to reject the promise if playback is not allowed in the current context, solving issues websites have to detect whether autoplay restrictions blocked the playback.
Docs: https://developers.google.com/web/updates/2016/03/play-returns-promise
Samples: https://googlechrome.github.io/samples/play-return-promise/index.html