As every tutorial shows, detail brushes don't add up to the VIS calculations, speeding up VIS stage and making maps run faster.
This stair is made up of detail brushes.
In game the result is this, triangles that the player will never see being rendered. But look! The part of the wall that is hidden under the stair is beeing rendered too, the lightmap is also covering that part of the wall, since the wall is one large surface.
Let's cut out half of the stair and texture with the common/caulk texture.
Now that half of the stair has been removed, the triangles under the stair are gone. With the additional benefit that the lightmap is gone in those surfaces too. Since the wall is no longer extending under the stair the lightmap of the wall's surface uses up a bit fewer pixels, which could mean less time compiling and a bit of space saved in the lightmap's bitmap.
A final note about caulk: on structural brushes it blocks visibility, but if that caulked surface is visible to the players, they will see a HOM effect, so caulk is opaque, not transparent.