But the SDB also has a hard steel, ground penetrating, front end, that can penetrate nearly two meters (six feet) of concrete. Not much use for that in urban warfare. But such a capability is very useful for taking out underground installations, particularly the entrances and air intakes.
I really haven't paid much attention to the weapon since I don't hear of it being used, but apparently the penetrating characteristic is what makes it important (as well as the small size which means planes can carry many more of these bombs), and it may never have been intended for city combat. So who's it for? Why, worried Iranians and North Koreans.
For the Iranians, they have nuclear inftrastrucure that could be hit with swarms of these delivered initially by stealthy B-2s and F-22s (and later F-35s) and by non-stealthy aircraft in subsequent waves. North Korea has the nuclear stuff as well as lots of buried air field hangars (which may house ballistic missiles, too) and artillery bunkers that loom over Seoul.
We may hope to protect Seoul by using SDBs to knock out all those artillery pieces in the opening hours of a war. Of course, we've been promised this result before in the relatively brief history of air power. I still assume we'd (well, more specifically, the South Koreans) need to carve out a no-launch zone to protect Seoul. but who knows? Maybe this time, for sure.
But this post also answered my long puzzling question of why we needed to build bombs capable of penetrating all the way down to a bunker no matter how deep it is in order to destroy it? Why, I wondered, can't we blow in the entrance and effectively destroy it by denying entry or exit? Apparently, the SDB can do just that.
NOTE: I added the link to the Strategypage quote, which I forgot.