It's double STAB rock attacks.
It may not be super useful now (better options for most typings, he's weak to Ice) but it could be useful against some Pokemon in gen 2? Not sure though...
I think getting attacks the same type on Golem is better than the alternative. He's obviously not a defender due to double water/grass weakness yet a lot of his defensive sets are rated A. I think he's an attacker so you want either double ground or double rock. Stone Edge would have been better IMO though.
