Who doesn’t like stacking* stuff once in a while?
In photography it has several uses. HDR photography for example is a form of stacking several exposures. The same can be done with the focus.
In macro or even more in micro photography this might be essential. I made quick example with the compound eye of a fly
stacked image
original image 1 of 3
Notice the difference in DOF? It seems almost negligible. However, with the right technique and an endless patience I could get the whole fly in focus.
The photos are made with another stacking technique: lens stacking. It is almost exactly what you think it is. The only thing is that one of the stacked lenses is mounted reversed on the other. 
stacked
The old nikkor 50mm 1.8 fits nicely on front of the tamron 90mm. And since the front element of the tamron is way back there’s no need to worry about scratches. You can also buy adapter rings to attach two lenses together. Mine is somewhere between Hong Kong and Arnhem right now.
The point being?
The whole point of lens stacking is to make micro( aka bigger than life size aka greater than 1:1 reproduction ) photos. So you can (actually have to) get really close to your subject. The downside? A depth of field of only 1mm! Or less. And that is exactly where the focus stacking comes in handy!
My advice, start stacking!
* A group of three rifles supporting each other, butt downward and forming a cone An orderly pile, especially one arranged in layers