Try to swap heads of two photos using layers.  Open up two pictures of two 
different people.  Cut off the head of one using the square region select and 
paste it as a layer into the other picture.  Now try to cut off the extra bits 
of the top layer so that the face can fit on the face of the other person.  You 
will find that in cutting off the extra bits you can either:

1. Make the cut off bits white
2. Make the cut off bits transparent (looks like gray and white squares)

This is not behavior you want as an editor.  You want to see the picture behind 
that you are revealing as you chop off the parts in the front so that you know 
what it looks like all together.  Sometime during these edits you will also 
notice that your layers have magically merged together into the background 
image.  As a user, I never told it to do so, yet that's exactly what it did.