Imagine you want to copy a piece of a scene you have done. Normally you would create a selection box and copy its voxel contents into the clipboard. The selection box can be resized, but so far, the selection boxes have been necessarily aligned to the world axes. What if you could rotate the selection box?
It does bring up an interesting possibility. Now you can get any slice of an existing object, no longer you are constrained to the horizontal or vertical.
The following video shows this in action:
In this case we were just rotating a selection box. The really nice bit about this is, it does not have to be a box. It can be any arbitrary volumetric shape.
This opens up a new set of tricks. You could for instance make a statue. Then select the statue using a regular on-grid selection box and copy it into the clipboard. Now comes the trick: you could use the clipboard contents (the statue) as the selection scope. This would copy the voxels as usual, but their outside shape will still conform to the original statue.
Another way to see it, is you can perform boolean operations using the clipboard. What I like about the approach is it feels simple once you try it. You do not need to understand the "booleaness" of it all. Just like you can have circular or even free-shaped selections in Photoshop, you can have your selection take any form you need. Sweet!