Warship construction is a lengthy, complicated business. The pre-construction phase alone - original inception, funding, design, creation of production facilities, training of personnel, and requisition of materials - can, for a large ship, take years; the actual construction of the vessel usually isn't much faster. The expenses are excessive, in both money and labor.
As one might expect, the pressure on the ship designers and architects is enormous; the bureaucratic infighting and political wheeling, dealing, and budgetary battling is unbelievable. Once a project is approved and work on the vessel begun, the Empire is committing itself to that vessel for the next several decades. At that point, any changes - even trivial ones - in the vessel's design can cost literally billions of credits and thousands of extra man-hours.
When Lira Wessex, daughter of the designer of the Victory-class Star Destroyer, proposed that the Empire produce the Imperial-class Star Destroyer, the angry debate between the Navy's military strategists, the Imperial Military Oversight Commission, and the Senate Budgetary Committee (since disbanded) almost destroyed the Empire. Some believe the Imperial too expensive, some believed it too unwieldily, others quite simply thought it was an engineering impossibility. The Navy loved it, of course; and through a combination of bribes, political pressure, and a rash of mysteriously crushed tracheas, it slowly brought the others into line.
Years later, when the first Imperial-class ship lumbered out of drydock (only 50 million credits over budget) and assumed active duty, the Navy was proven correct in its belief.
The Imperial-class Star Destroyer has enough firepower to reduce a civilization to slag or take on a fleet of enemy vessels. Each carries a full stormtrooper division, complete with assault craft and ablative heat-shield for orbital drops, 20 AT-AT and 30 AT-ST walkers for ground assault, and six TIE fighter squadrons for escort and patrol.
There are whole star systems whose gross domestic product is less than the cost of a single Star Destroyer. There are whole nations which, throughout their entire history, do not use as much energy as a Star Destroyer expends to make a single hyperspace jump.
The galaxy is huge; even the Empire has never visited a majority of the stars within it. Controlling it all is impossible. The best that can be hoped for is to frighten all into submission with the threat of destruction - and swiftly and ruthlessly crush any opposition that appears. The Imperial Star Destroyers are the weapons the Emperor uses to rule the galaxy. By deploying a Star Destroyer and support ships to a system, the Empire can destroy virtually any foe.
It would be impossible to garrison every system in the Empire - but its Star Destroyers give the Empire the ability to project its power anywhere it wishes on short notice. Much of the Imperial Navy is permanently deployed in reserve in the Galactic Core, ready to swiftly respond to any threats, anywhere.
The Imperial Navy is organized into fleets, which are composed of one Imperial-class Star Destroyer accompanied by support and lesser combat ships. Each fleet can operate independently. In practice, Star Destroyers can be combined in Task Forces of three, Sector Squadrons of six, or Regional and Territorial Fleets of 24 - but it is unusual for more than one to occupy the same star system, except for ceremonial purposes. Such concentrations of force are rarely necessary - there are few things which can seriously challenge even one Star Destroyer.
An Imperial Star Destroyer is more than a weapons platform: because of the variety of its duties - planetary defense, planetary assault, and ship-to-ship combat - it must also be something of a space station, repair dock, and heavy transport as well. In addition to combat craft, an Imperial Star Destroyer carries eight Lambda-class shuttlecraft and many repair and recovery vehicles. Since they are not designed to enter planetary atmospheres, each Star Destroyer also carries 12 landing barges for crew and cargo transfer and for ground assault. Landing barges are shielded and armed, and carry up to four AT-AT's (or eight AT-STs) and 1,000 troops each. They can also haul the heavy weapons, supplies, and equipment required for planetary operations. Many Star Destroyers carry complete pre-fabricated ground bases, which can be dropped and installed within days. When a base is deployed, the Star Destroyer dispatches 800 troops, 2,200 support personnel, 10 AT-ATs, 10 AT-STs, and 40 TIE fighters as a garrison.
Of course, space combat is their main duty, and Star Destroyers are well designed for it. Turbolasers and ion cannons are mounted in five-gun batteries. Each battery contains three turrets, two of which are double-mounted, and one single-mounted. The turrets can jointly target a single enemy ship to concentrate firepower, or fire independently to engage multiple targets.
However, the placement of the batteries is a weakness: there were few overlapping fields of fire at close range. While this is no great problem in actions against larger capital ship opponents, it is a drawback when fighting small and highly maneuverable ships, which can dart under the guns.
Naturally, most Imperial Star Destroyers carry TIE fighters to deal with small opponents; this fiscal year's Imperial budget provides funds to retrofit the remainder with TIE flight decks. However, in light of the disaster at Yavin, funding may be diverted to more vital purposes.