No, it hasn't come out what that button does.
I would guess that the user can set up a number of faces that they can then quickly switch between. Ie, there may be 6 different face types, but the user can set up a bunch of different faces with different complications and then switch between them without having to set each one up again.
Again, just a guess...
I'm assuming you can use that button to purchase more watchfaces from Apple.
Think you can use it to create your own watch faces with your own photos too.
Seen in a few videos where they have a picture of some people and the time overlaid onto the photo