Brick rakes, as their name suggests, are designed to be used on brick walls.
Their wheels run along the brick surface, dragging the pin along the mortar, scraping it away.
Bricks are fairly even on the surface and so running the tool along the face of these ensures an even depth clearance of mortar.
Brick rakes are useful when you do not wish to run the risk of chipping the brickwork, which can happen when using a chisel.
It is useful to use a brick rake when power tools are not an option – for example on listed period property, where you need to be extra careful not to damage the brick face surfaces.
You would use a brick rake when you wanted depth control. You can set the raking out depth using the spade bolt and the robust pin, unlike a plugging chisel where the depth varies due to human error.
When not to use one
You would not use a brick rake on a stone wall as the stone surface would be too uneven to run the wheels along and the pin may not be able to reach the mortar to remove it.