The V.I. Lenin Nuclear Power Plant, also known as Chernobyl, was a massive power-generating installation with four RBMK-1000 nuclear reactors. Its primary purpose was to provide electricity.

The RBMK reactor, however, was a very unusual and very Soviet design. Nuclear reactors can be engineered for different kinds of applications and within different kinds of constraints. The Pressurized Water Reactor (PWR) design common in the West, for example, was originally developed to be the reactor for nuclear submarines and nuclear aircraft carriers, and so was designed to be something that you could get a lot of power out of a small size if you needed to. By comparison, the nuclear reactors at Hanford and Savannah River were designed to primarily produce plutonium, and that means that they were designed so that fuel could be cycled through the reactor very quickly (which decreases the build-up of byproducts that make the plutonium less useful for weapons). You can even design reactors that can absolutely have no chance of a nuclear meltdown, like the TRIGA research reactor, which because of a clever set of choices in materials slows its reaction the hotter it gets (and so cannot overheat).

The RBMK, by contrast with the above, was designed with a few priorities in mind. The highest was being economical in operation and construction. Nuclear reactors have very high capital costs, and many designs (like the aforementioned PWR) require specialized factories to build things like the specialized reactor pressure vessels, which then must be shipped to wherever the reactor is being built. The Soviets wanted a reactor that could be essentially constructed entirely locally with non-specialized (at least, not specialized to nuclear) labor forces and production lines. So the RBMK was designed as a reactor that could be built with local labor and facilities — think of a reactor vessel that is more concrete than steel.

The agency in the USSR that designed all reactors was the same on that ran the nuclear weapons program. (By comparison, the agency that operated the reactors was the one that operated other kinds of civilian power sources.) So it is also interesting that the RMBK was designed so that, if you wanted to, you could also produce plutonium with it. As noted above, that means being able to cycle fuel through the reactor much faster than you would in a power reactor. There is no evidence that the Soviets ever used their RBMKs for this — they had other reactors for producing plutonium at Soviet nuclear weapons plants — but they designed the reactor to "hold the door open" on the idea that sometime in the future they might need a lot of plutonium and they could divert some of their electrical infrastructure to making it.

So this might be what the other Redditor had in mind, I don't know. The Chernobyl nuclear power plant was not a military installation and was not run by the military, and it was not used to produce plutonium. But the type of reactor it was, the RMBK, could be used towards this purpose. So it is exactly backwards: energy was the primary function; plutonium was a secondary option, but one never used there.

The RBMK design was not one that had safety as one of its major priorities, which is somewhat evident in how things went down and some of the major shortcomings of the reactor (like not having a dedicated containment dome, which would increase the cost). It was optimized to very specific, very Soviet circumstances. (The Soviets also had an equivalent of the PWR reactor, the VVER, which many of their other plants used.)

The best book I know about the development of the RMBK, is Sonja Schmid, Producing Power: The Pre-Chernobyl History of the Soviet Nuclear Industry (MIT Press, 2015), which also gets deep into the organization of the Soviet nuclear industry and probes at some of the systemic issues that led to the Chernobyl accident.