Even in an era of nuclear tipped missiles of global range, the seemingly humble nuclear gravity bomb retains an important deterrent role in that it can be carried by a wide range of aircraft that have routine non-nuclear missions. Thus no-goodniks like China or Iran, for example, have to wonder just what a handful of B-2s deployed to Diego Garcia might be up to. When a US general or diplomat warns that the US is “prepared for all eventualities” it means that conventional and nuclear options are available to the US without risking a general nuclear war by launching land of seaborne ICBMs.
I’m very sure that using a nuclear bomb on China or Russia in any capacity would escalate into a full exchange pretty quick.