2.8 update: jump to this post
This add-on hides the backfacing or non-visible geometry to address the issue of Backface Culling not hiding mesh components in Edit mode (vertices, edges, faces), as pointed out by this rightclickselect proposal.
It is mostly useful for retopology with X-Ray mode, but isn’t limited to this use case.
There is also a version for blender 2.7x
This image shows a comparison between the built-in Backface culling and the Backface hiding from this add-on when doing retopology on top of a high-poly mesh with X-ray enabled on the low poly mesh :