I think it's just to stop the initial suction so it wont dry to quickly, if you apply sbr you could do all the stones then lay them. If you use water then you will have to damp every stone down as your going to make sure they all stay damp as there being layed.
If you use a feb in your sand and cement mix also it will keep it more plyable for longer

In my opinion the answer is yes !it only takes seconds to do each slab give's better adhesion /blood bridge from slab to mortar ,can also stop any mortar staining seeping through to top face of slab .
Thanks
Chris

In my personal opinion as long as you have a good solid level bed of mortar and is of a damp consistency but not to wet i brush the back with water on all the Indian sandstone being bedded, you can lay straight down, never had a problem..

With Indian sand stone and any natural
Portland, forest of dean etc, I always brush a cement slurry to the bed of the slab either white or grey cement depending the stone used. Make sure the bed is full not a dot and dap.

Firstly sandstone, whether Indian sandstone or not sandstone will vary in its porosity, and ideally a sandstone with low porosity, is preferred for external paving, I would personally apply a bonding coat of PVA and cement to guarantee a bond between bedding material and stone especially on thinner materials, as they are prone to popping free, and of course a full bed, no mortar dabs.

Depends on the current climate. As it's currently hot we normally give thinner slabs, like indian sandstone, a quick brush with a pva and water mix. Helps with suction as previously mentioned by another comment. On thicker slabs we have scored the underneath as well as pva'd them before for extra grip. Works, no complaints.