I think i figured out something (probably there could be more to it). Basically, it is a new way to keep the permissions within the context of the page which i feel is nicer way to handle instead of the traditional classic UI user admin console where it often leads to confusion.
A simple use case to see it working
1) Logged in as admin, create a custom user and assign it to a custom group or the existing group(authors, contributors)
2) Pick up a page in geometrixx(hiking) and edit the permissions by adding a new permission. In my case, I selected all the permissions( browse page, publish/unpublish, create sub page) but for the 'delete page'
3) Now, login with the custom user and you will see that you can not delete the page(hiking) where as you could delete all the other pages.
So, yeah a nice feature to manage permissions at a page level.