For the 416 i can see the point, since they seem lighter and more comfortable to hold than the standard HK forearm. Plus the 416 version is made to work with the HK upper (flattop-rail sits higher than on a regular AR) and most other rails dont.
For a regular AR, i dont see the advantages either, except for the 416-style barrelnut maybe. Looks quite solid and you dont have to bother with gastube-alignment anymore, but i guess in AS that doesnt really matter
They sure look good though..
Yup and in one of the last replies the author confirms that the rail fits a VFC 4