Q:

What is the significance of the off the shelf AEM user group called everyone? 

In AEM, when we create a new user using the useradmin console, the newly created user gets added to an off the shelf user-group called everyone .  What is the significance of the off the shelf group - everyone ? 

A:

It's the easiest way to address all users of an AEM instance.  
I think that's all there is about this group.  It is just there to enable you to set ACLs that are used for everybody.