To expand a bit:
Fixed mounts, i.e. mounting the gun directly on the hull, limits your tracking speed to either
a) the speed of the ship or
b) your researched basic beam weapon tracking speed
whichever is larger.
example:
You have a laser destroyer, capable of 3.000 km/s and have researched 4.000 km/s tracking speed
Your fixed mount lasers will have a tracking speed of 4.000 km/s
If your DD would be capable of 5.000 km/s, this would be the tracking speed of your lasers.
As long as the targets, you want to shoot at, are not significantely faster than your ships _or_ your basic tracking speed, you can simply slap your guns on the hull.
If, however, you want to engage fater enemies (FACs/Fighters or missiles, for example) turrets come into their own right.
Trying to hit a missile, going at 20.000 km/s, when your guns can only track at 4.000 km/s is kind of hard.
Turrets can be build with any tracking speed you want. Of course, the faster you want the turret to track, the more massive it gets.
Your beam targeting systems are limited to 4 x basic tracking speed, however.
Personally, I design my turrets with 4 x next tech basic tracking speed (i.e. I have 3.000km/s researched, and my turrets will be build with 16.000 km/s tracking speed, as 4.000 km/s is the next level of beam tracking speed). Once I get that tech, I will only have to upgrade the beam firecon and can leave the turrets in place, saving a lot on the upgrade.
Note: A turret with a tracking speed of 30.000 km/s will do you no good, when your beam firecon can only track at 12.000 km/s as the slower of the two is what counts.
