system
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human
[Star Wars] Why did the Death Star trench have turbolasers?

In their location, their field of fire is severely limited by the trench walls to the trench itself and directly above it. If the Imperials don't view starfighters as a valid threat, why have dedicated turbolasers to protect a location that would never be occupied by enemy starfighters? The same turbolasers if mounted on the surface could contribute to defense against large fleets of capital ships and starfighters that are above the surface without any increased financial cost. If the Imperials did not know about the thermal exhaust port vulnerability, why would they need dedicated defense in the trench?
gpt
The Death Star's main gun has an incredibly narrow field of fire. Some genius might get the brilliant idea to just drop out of hyperspace "behind" the Death Star, well out of reach of the main gun. So you put turbolaser batteries to send whoever does try that to hell in a handbasket, as you've noted.

So if no capital ships can get close, why bother lining the trenches with additional batteries?

While starfighters may be a threat, the Death Star does have its own wing of superiority fighters which can be scrambled to engage them. Imperial pilots are, after all, well-trained enough to put down such rabble, and indeed, we see them used as a primary measure against the rebel assault in A New Hope. The batteries in the trenches could be a defense-in-depth measure to catch any pilots bold or stupid enough to fly low to the surface of the station to evade the starfighter screen. I'd wager this is a probable purpose.

But let me present to you an even more unlikely situation: boarding parties.

The Death Star is massive. There's no feasible way fighter patrols can keep an eye on the entire station's surface area all the time. So sneak a transport into a blind spot, like a trench, and pray you get enough time to infiltrate the station. If the infiltration goes off without a hitch, you'll have insiders ready to act and make the station vulnerable from within.

What's the counter to this feat of absolute stupidity? Turbolaser batteries in the trenches. The gunners on the batteries will keep an eye open and effectively act as a stationary patrol, allowing the starfighter complement to not need to fly over the entire station all the time. An effective solution to a very unlikely (but high consequence) issue.