Before the advent of computer-aided bombing, it wasn't easy to hit a target with a bomb even if the intended target was not maneuvering. In order to account for the ballistic drop of the bomb after release, the bomb had to be dropped at a specific combination of airspeed, altitude, and dive/descent angle. Because of the degree of difficulty in having all three nailed at the same time as well as having the target centered in the sight (particularly while being shot at), the pilot had to drop at a higher altitude if he was a little fast and at lower a lower one if his dive was a little shallow. And if the drop was a fraction of a second late or early, the accuracy decreased. Not to mention the presence of wind aloft that would affect the fall of the bomb.
It was therefore necessary to practice. Since prewar carrier-based airplanes were intended to carry heavy bombs in order to do the most damage to an enemy ship or shore installation, only three bomb pylons were generally provided. To get the most effective training from a flight, practice bomb dispensers were therefore developed. These were the original Multiple Ejector Racks. The Mk 42 was a plate on which three practice bombs could be loaded. The more widely used Mk 47 was a streamlined dispenser that was about 40" long.
It contained compartments for eight miniature bomb-shaped weights.
Navy fighter planes have always been used for bombing as well. The number of aircraft that can be loaded on a carrier is finite so it is important that fighters can drop bombs if necessary. Fighter pilots trained with the practice bomb dispensers as well (this one on an early Corsair looks like it might have only had six compartments).
Rockets were somewhat easier to aim, easier even than guns or cannon because they were propelled in flight and therefore did not go ballistic after being released. As a result, a pilot only needed to fire a few at the expected airspeed, altitude, and descent angle to be used in combat and note where they hit relative to his aiming point. Accuracy was affected of course by the wind, any sideslip (if the airplane wasn't heading the same way it was going, the rockets would tend to align themselves with the relative wind immediately after launch), and any damage to or misalignment of the fins. The Sub Caliber Aircraft Rocket (SCAR) was developed for training to reduce the cost of expended rockets.
It was 2.25 inches in diameter and about 29 inches long with a tail-fin span of 8.3". It could be configured to imitate either the 3.5 inch or 5.0 inch rocket with the result that some were about six inches longer. Adapters were provided for the different rocket pylons that were used.
A rather bulky camera could be mounted externally on the aircraft for training flights to provide feedback to the pilot on his technique and accuracy. This was particularly useful for torpedo bomber pilots who couldn't make multiple drops on their training flights (or any drops at all due to the expense of torpedoes).
Thanks to David Collier for suggesting the topic.