system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] why develop a Death Star when destroyers can wipe out a planet like Taris?

This is the only instance of orbital bombardment I can think of, and maybe it’s more difficult than we see, but it’s damn effective. Saul Karath implies that ‘billions’ on the surface will die. News of Taris’ destruction is spread across the galaxy, so the tactic of fear is still really effective, even if the planet itself is intact.
gpt
So there's 3 answers to this.

The canon answer is the death star serves as both a mobile throne for Palpatine and it's time efficient to use. It destroys a planet in one shot and in minutes whereas a fleet takes hours or even days to scour a planet. Not to mention how long it takes to amass that many ships. 

The legends answer is the death star was built to fight the yuuzan vong. Palps needed a powerful weapon capable of quickly decimating fleets and creating a fire line of scorched earth planets to deny the vong a foothold in the galaxy. The death star could fill this purpose along with it's more refined successors in the eclipse and world devastators.

The infinites answer is Palpatine wanted multiple death stars to be able to threaten every system in the galaxy with nigh unkillable stations that could erase their planets. The imperial navy still had a purpose but would be the patrol force of his empire to the death stars' threat deterrents. 

And taris was unique in that it's city was stacked upon itself. Causing enough destruction on the surface would cause a collapse of lower levels as supports would fail. The city would crumble under it's own weight leading to further destruction and the resulting damages would trap most of the population under billions of tons of rubble to die from starvation and suffocating as relief efforts would have to unearth them. It's easier to destroy a planet from the surface if it's all one big house of cards than to destroy spread out settlements and cities like Alderaan had.