Hi Paul,

The wooden pattress, in my experience, has always been used when mounting a surface mount switch or socket, like the one shown in the picture, to a plaster or masonry wall.

[Linked Image from ts.smoothcorp.com]

The reasoning is that you would use larger and longer screws (or nails) to anchor the block to the wall and then use the tiny wood screws that normally come with the switch or socket to fasten it to the pattress.

It makes for a much more secure installation than just using small wood screws to directly anchor the switch to the plaster wall. That way the device won't pull away from the wall when used.