To make bombs. And what's changed now is we have enough bombs.
Seriously, even the reactors that aren't expressly breeding fuel were designed with an eye to the technology and as part of programs in the 50's-70's to build out expertise in this critical technology area of national security.
Countries wanted bombs. To make bombs you need a nuclear industry. Ergo, everyone who wanted bombs built civilian reactors, without exception.
This argument doesn’t hold. There are several countries such as Finland that have operated nuclear power plants for decades and do not have ambitions for nuclear bombs.
If you want to go with that argument: cite Japan, not Finland. But it doesn't have to be 100% true to be largely true. The overwhelming majority of nuclear capacity has been built out by nuclear powers or aspiring nuclear powers. And with few exceptions the end of their warhead buildout happens to correlate with the end of their megawatt buildout.
Japan has a stockpile of separated reactor grade plutonium sufficient to make thousands of bombs. This is a deniable kind of proliferation. If push came to shove they could weaponize that material without having to make new plutonium.
(And, yes, reactor grade Pu CAN be used in weapons, with proper design.)
I think a lot of countries built reactors when the economics of Nuclear reactors as well as the economics of renewables were unknown with respect what they would look like in scaled deployment. Modern looks at the operating costs vs time to build, vs modern construction practices from the US, France, China, et al seems say the economics of Nuclear reactors have stalled or increased, while the economics of renewables continue to make accelerating improvements.
