Useful for bump map baking where a consistent triangulation should be enforced when baking/exporting/importing, to avoid artifacts caused by a different triangulation of the mesh by that which was used for baking by internal/external tools.
documentation is here http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.65/More_Features
Will probably add some pictures too to demonstrate the issue that is solved more clearly.
Currently using the skin modifier icon, will soon change that.
Review by Brecht, thanks!