Why does the Erling brick (#4070) have a ridge below its outward facing stud?
Whereas the more versatile and less common #87087 does not? The #87087 piece also has a better use in SNOT usage, as it can accomadate larger bricks.
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The ridge is there to make the base the standard 5LUs wide (see also "What is an Erling Brick"), enabling it to accept the standard stud.
I believe the rest of the brick was thinned out so that the combined "depth" of the Erling brick and a 1x1 plate (its common usage as a headlight) was the same height as a standard brick.
If headlight bricks are turned over by 90° and a plate is attached to the stud then this construction is as high as a standard LEGO brick or a layer of three standard LEGO plates.
It also helps to keep the 1x1 plate straight to the bricks edge.