Currently the main reason to deploy
enhanced radiation warheads is roleplay - they are only really useful if you want to make an NPR home world glow in the dark and watch its surviving population fall into Mad Max anarchy.
However, one of the original use cases for the development of the neutron bomb (the original enhanced radiation warhead) was as a tactical weapon in support of ground forces - the bomb would cause reduced physical collateral damage while still irradiating enemy combatants. From this
1981 article:
The long years of engineering grew out of a notion originated by Samuel T. Cohen, a Defense Department consultant, in the mid-1950’s. Around 1957, at the instigation of Edward Teller at the Lawrence Livermore weapons laboratory, work began that led to the development of a device which, according to Defense Secretary Caspar W. Weinberger, ”enables infantry to fight closely behind it, as with conventional artillery.” Army experts interviewed at the Pentagon admit that the Secretary is exaggerating. But they do believe that the neutron bomb could be used in a way that would cause less collateral damage and radioactive contamination than standard fission weapons. General Fulwyler says, ”The enhanced radiation weapons offer even greater possibilities for use than the weapons of the past.”
The chief Congressional proponent of the neutron-bomb plan is Samuel S. Stratton, a Democratic Representative from upstate New York. ”The tactical nuclear weapons that we have at present,” he says, ”are 10 kilotons. You need 10 kilotons to destroy a tank. A neutron weapon is one kiloton, and you can explode it without touching the ground. As a result, there is no fallout whatsoever. All you have to do is be in a basement away from the immediate blast and you’re safe.” While Representative Stratton’s technical brief would appear to be at odds with Pentagon statements about how the bomb would be used and scientific analyses of its effects, his political logic has proved powerful in Congressional debates. ”The neutron weapon is essentially defensive, simply because it kills tank operators without destroying the German countryside you’re trying to defend,” he says. ”A weapon that is primarily limited to just killing soldiers and goes out of its way to preserve the invaded territory is offensively useless. The Russians are opposed to it because they cannot profit from it even if they could build it.”
I think that one way to improve the appeal of ER warheads could be specialize them into the planetary bombardment niche. One way to do this could be to make ground forces suffer damage from warheads' explosions
and their radiation output. "All you have to do is be in a basement away from the immediate blast and you’re safe” could presumably be simulated by making GU fortification extra-effective against radiation attack.