system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] I have read that a handful of star destroyers could kill all life on a planet, and render its surface uninhabitable within a matter of hours. If that's true then what's the point of a Death Star?
gpt
So, first of all there are some caveats to those feats. Although three Imperial-class Star Destroyers could execute a Base Delta Zero (total annihilation of everything on a planetary surface, and much of the upper crust to a molten slag) in "a matter of hours", there were also planetary shields that could hold off such bombardments for a sustained period of time. Time, in which enemy reinforcements could be called in, evacuations could be attempted, and things could go generally wrong.

But even more important than that was the sheer implied power of the Death Star. It didn't just "atomize the surface" as ISDs could, it overwhelmed a planet's shields and blew its  apart the entire mass apart instantaneously.

The entire Tarkin Doctrine was based on the implied power of Imperial Forces, and the fear they could induce in a target. This, at times, called for production of vessels which were not always the absolute best strategic option but carried strong psychological impact in them. The Death Star was one such result.