Humans can improvise under pressure. Computers can't.
Imagine you're on the bridge of a large capital ship, traveling with an escort to a nearby star system. You're being deployed to provide support for a planet under siege by the enemy. On the way there, you run into several wings of fighters, which line of to make attacking runs. If they can take out your ship, the planet will fall and they can win the siege.
There are a few different ways this could play out.
Why you don't use a central computer:
First officer: "Captain, we've got a wing approaching from the port side. They're lining up for a strafing run."
Captain: "Re-align all plasmonic laser batteries."
Central AI gunner: "Laser batteries aligned to port. All guns ready to fire on command."
First officer: "Here comes the first squadron."
Captain: "Bring the tracking system on line."
Central AI gunner: "Tracking system ready. Targets sighted."
Captain: "On my command. . ."
The entire ship shakes. Several explosions are heard.
Captain: "What in the name of Zarquon happened?"
First officer: "We've been hit from behind! A few stray fighters got past the escort and took out the central computer center! The guns are offline!"
Captain: "Can we switch to manual, or get local command of the circuits?"
First officer: "We have nobody who can fire the damn things! They aren't designed to be fired by hand!"
The captain tries to say something, but the bridge is taken out under heavy fire from the first squadron.
The problem with having one central command computer is that if it's taken out, the entire ship is unarmed. All the enemy has to do is go for that central spot - in this case, the area near the back of the ship, adjacent to the bridge - and the ship is absolutely helpless. Yeah, you can build in redundancy and backup systems, but one hit will still destroy the system. That should imply that you should use different computers at each gun, right? . . .
Why you don't use individual computers:
First officer: "Captain, we've got a wing approaching from the port side. They're lining up for a strafing run."
Captain: "Ensign, send a signal to the system to re-align all plasmonic laser batteries."
Ensign at gunning command terminal: "I've set the laser batteries aligned to port. I can confirm that all guns are responsive, and ready to fire on command."
First officer: "Here comes the first squadron."
Captain: "Bring the tracking system on line."
Ensign: "Tracking system ready. I've deployed three batteries on each of the incoming fighters."
Captain: "On my command. . ."
The approaching squadron gets nearer. Those on the bridge can see it resolve itself into six shapes, which suddenly break formation.
Captain: "Have the computers fire at will!"
Ensign: "Command sent."
For a minute or so, the plasmonic batteries blaze away as the six fighters dodge the lasers, strafing the ship when possible. Several escort fighters lock onto them; one spirals towards the front of the ship and crashes into it, exploding. The bridge shakes.
Captain: "What in the name of Zarquon happened?"
First officer: "We've been hit!"
Ensign: "I've lost all communication with the Sector D and E batteries. They're unresponsive."
Captain: "But the fighter hit only part of Sector E!"
Ensign: "Half of the batteries in Sector E were destroyed outright, but the targeting systems and communication lines were lost with the rest in that Sector and all in Sector D. They should still be fireable, but their controlling computers are virtually destroyed."
Captain: "Can we reroute control from other computer batteries and have them work simultaneously?"
Ensign: "No. They're overloaded, and at any rate, there's no communication. We can't move resources around right now!"
The captain tries to say something, but the bridge is taken out under heavy fire from the first squadron.
The problem with having local computers is that if part of the targeting systems are hit, it can be difficult to reroute command. The computers "talk" to one another via circuitry, and receive their commands the same way. If part of the ship is damaged, it could - if designed just the right/wrong way - mean that an entire section of batteries are useless.
Why you use humans:
First officer: "Captain, we've got a wing approaching from the port side. They're lining up for a strafing run."
Captain: "Lieutenant, signal the gunners to align their sights with the approaching squadron."
Lieutenant: "Unit 1, align all batteries to port. Set up your tracking systems. Over."
Voice over radio: "Batteries aligned to port. Trackers on. Over."
The lieutenant repeats this several times while the fighters get closer.
First officer: "Here comes the first squadron."
Captain: "On my command. . ."
The approaching squadron gets nearer. Those on the bridge can see it resolve itself into six shapes, which suddenly break formation.
Captain: "Fire at will!"
Lieutenant: "Fire at will!"
For a minute or so, the plasmonic batteries blaze away as the six fighters dodge the lasers, strafing the ship when possible. Several escort fighters lock onto them; one spirals towards the front of the ship and crashes into it, exploding. The bridge shakes.
Captain: "What in the name of Zarquon happened?"
First officer: "We've been hit!"
Lieutenant: "I've lost all communication with the Sector D and E batteries. They're unresponsive."
Captain: "But the fighter hit only part of Sector E!"
Lieutenant: "Half of the batteries in Sector E were destroyed outright, but the targeting systems and communication lines were lost with the rest in that Sector and all in Sector D. They should still be fireable, but it looks like the gunners were killed by the fireball and guns from the fighters."
Captain: Can we send anyone down there?"
First officer: "Yes. We have a dozen men on reserve down in the loading bay who were to prepare for a surface mission."
Captain: "Have them go to fill in as many batteries as possible. I want those fighters brought down!"
First officer and lieutenant: "Yes, sir!"
Humans are great in situations where you have to improvise. In this case, a substantial amount of gunners were killed from laser fire and an exploding fire, leaving the systems running but with nobody controlling them. The captain and officers were able to guess this, and sent men down there to replace them, even though that part of the ship was heavily damaged. Several batteries could have been partially damaged - heck, the targeting systems could have been entirely brought down - but I think human improvisation and spur-of-the-moment decisions could lead to a victory.