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.
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.
It also helps to keep the 1x1 plate straight to the bricks edge.