Cover the bases: you can't be sure someone else won't discover a new type of nuclear weapon or something you haven't thought of. The big one would be if someone figured out how to get kiloton->megaton yields, from a weapon which didn't produce radioactive fallout.

Sure, target a city and it's functionally indistinguishable, but what about against military formations or bunkers? It would be a big game changer if someone eliminated the fallout problem, since it changes how one could plan to use nukes in a defensive posture.

So you do the research to make sure you're always within a year or two of whatever the next state-of-the-art will be. Being the dominant hegemon has a lot of cost-of-living expenses.