Google created a mild amount of confusion when it launched its Home speaker. You could say "hey, Google" to start a command with the living room device, but you still had to use the time-honored "OK, Google" on your Android phone. Needless to say, that could be confusing if you used both platforms. However, Google is finally sorting things out. Many Android phone users have reported that Assistant is asking them to reconfigure the voice modeling, and is giving them a choice between "hey, Google" or "OK, Google" afterward. You can embrace consistency across devices or stick to tradition, in other words.
The extra choice seems like a simple addition, and it won't please everyone (why not let users pick a custom hotword?) but it's potentially very important. As anyone who has ever tried using multiple voice assistants can tell you, having to remember multiple trigger phrases can be a headache. The "hey, Google" option reduces the chances of a slip-up, and might just avoid some frustration.