Beta game has unreleased features and gets events early. Right now the football bowl (an event) is running on beta.
Right now I don't think there are any major unreleased features running on beta right now but there will be in the future. They recently tested daily challenges for months before we got them, as well as having the new autobattle feature and the kraken counter earlier than we did.
Same for Oceanic Future part 4, 3, etc - Beta gets it all first. You get free diamonds for playing on beta (only useable on beta server) as a thank you for testing things out, so it can be a fun place to play.