Who? Whoever can build them. Why? It makes it immensely more difficult for other countries to screw with you.
Who? Whoever is trusted by powerful friends to have them, or are powerful enough to otherwise fight those who do, and has serious reason to believe they’ll be needed.
Why? Odds of abuse, or lack of need, equals no nukes for you; responsible & necessary ownership allows. North Korea and Iran are making it very clear they want to kill people with nukes for the he11 of it, so will likely find out the hard way that the Big Boys will disallow it. Saddam’s Iraq was violently overthrown when it looked like he was building nukes, after invading neighbors. Syria gets its nuclear program bombed to smithereens periodically by Israel. India & Pakistan both have nukes, but aren’t going to molest anyone besides each other, which they won’t because of their nuclear standoff. Ukraine found itself a nuclear superpower by accident, and quickly gave them away lest it quickly become a target. Nobody is going to nuke Canada, who relies on the USA to provide such protection in that extremely unlikely case. All of Israel’s neighbors want to obliterate it yesterday, so the USA has quietly furnished nukes as appropriate as deterrent. And the big 3 nuke owners have so many they’re all “f u” to anyone saying they shouldn’t.
If we DON’T manage to slap down, undermine, or otherwise remove N. Korea from the nuclear club, and prevent actors like Iran from getting there, the odds of a very significant nuclear war (100 or more warheads detonated) within the next 50-60 years go up drastically. Then it’s at least a 50/50 chance. The odds of a smaller nuclear exchange are even higher. If it’s not the Norks, it will eventually be the Iranians. Or the Egyptians. Or the Saudis. Or maybe someone not yet obvious. Only the details will differ.
Since I’m not likely to live more than another 25-30 years, I may not “see it”, if we fail. But the odds of my grandkids getting, at the least, radiation sickness, are another story.
Just look at what N. Korea is saying: That their lower level officers are authorized to strike the instant the first U.S. (etc.) weapon hits them. Never mind, for the moment, that this is the only rational plan they can have* (if they wish to continue in their blackmailing ways): The odds of a mistake or too hasty response are sky high.
*The Norks surely know that the US can largely decapitate their high level leadership, so, the “appropriate” deterrent is to make sure the response to a US attack is very fast and does not depend on top level authorization after the USD attack begins. In effect, the Norks have to fire away as soon as they detect, or think they detect, attacks from as close as our subs parked offshore, so they (the Norks) have only minutes to launch themselves. Otherwise, by the time they check with Pyongyang, both they and Pyongyang are rubble, or worse.
Now, I doubt the Norks can presently launch multiple nukes on that kind of short notice. I also doubt they will fail to get there, given a little more time.
As a good friend of mine, an engineer who once worked for TI’s defense products division, told me many years ago: “Even ‘Rocket Science’ isn’t really ‘rocket science’ anymore.”
“Given North Korea’s nuclear lunacy, what exactly are the rules, formal or implicit, about which nations can have nuclear weapons and which cannot?”
Easy.
The two countries who have more the 7,000 nukes each, and 1500 each deployed for immediate launch...
Get to decide.
When they don’t agree, that’s when there are issues.