The Doomsday Clock is a judgement call on the likelihood of a global catastrophe. These days use of nuclear weapons would trigger a global catastrophe, but that hasn't always been the case.

Back when the global nuclear arsenal was a lot smaller, was delivered by aircraft instead of ballistic missiles, and the negative effects of nuclear war weren't well understood, nuclear weapons were still on the table for regular use in warfare.

If a war had broken out between the US and the Soviet Union in say, 1950, nuclear weapons would almost certainly have been used. Mutually assured destruction wasn't a thing then, and many people believed a war between the US and USSR was inevitable. And it would have been bad - but not nearly as bad as that same war if it had happened in 1980.

Plus there's the Cuban Missle Crisis. That was a wake-up call. Measures were taken afterward to ease back on the itchy trigger fingers after that.