Let's not quibble for the moment over how it came to be. Perhaps you decided the Reliable Replacement Warhead program was a good idea after all and grabbed one of the designs that were put forward in the twilight of Bush/dawn of Obama and built it. Perhaps you had a brand new one whipped up and it's gonna be great! All that matters is that you have it.
But does it actually work? The computer models say it's A+, hunky dory, best nuke ever. Except now it's a physical object, not a simulation. Were the engineering tolerances right? Did we get the metallurgy down? WILL IT ACTUALLY WORK?!?! Unless you can convince the brass that it's does, they won't order this new design, much less deploy it. And you won't know unless you set it off, as a representative of a new fleet of nuclear weapons. And so begins the Choose Your Own Nuclear Adventure!
Atmospheric testing does have its benefits, namely that it's comparatively easy to do and, by jingo, people will know that you set off a nuclear device. Very showy and attention getting, great media coverage I bet. Here's a few demonstration videos. There is this one slight drawback in that Kennedy kinda, sorta signed a treaty and Congress ratified it 54 years ago that we said we wouldn't do atmospheric testing anymore. That will make some people very unhappy. But you do you.
Let's say, for the sake of argument, that you have a brand new nuclear weapon.