Many apps have a similar feature, it is always useful to return to an earlier point. The Photo app does have a neat and super fast history panel but if you have a history panel full of commands all indicating add pixel or gaussian blur or transform it is actually quite hard to find exactly the correct point to return to, the snapshot feature makes it a whole lot easier. The key menu is document and 'add' and 'restore'
So you have added a number of layers and effects as well as adjustments and type and at that point you wish to save a current state / history state. Go to the add command in the document menu (saving it to the AP document format) and then give it a name though the default text is perfectly reasonable giving the current time etc as well as day. When you save the document, that added state is added to the document as well so if you close down the app etc and return to the document at a later time, it is still there.
How to restore, well that is the next menu command in the document menu. If you have used the add command multiple times or just once, you will see an entry in the restore menu with the time and date stamp or whatever text you added to the add. It is as simple as selecting one of those and all the various layers etc will be restored as before.
This is a very useful feature. If you have any history states stored then you can go to the layer menu and select the new layer from snapshot command and you will see a pixel layer appear that shows exactly what was displayed on the screen. It does not restore the actual layers (that is the restore command) but it does generate a pixel screen dump and that layer can then be manipulated in exactly the same way as any other pixel layer which can have opacity added as well as using blending modes or adding effects to the screen layer.
You can find out more about the add / restore etc [Youtube]