"The armorer's skill. Increases the efficencey of vehicles weaponry. Reduces the overheating speed of primary weapons, as well as spread of bombs and rockets for aircraft."
Machine guns and cannons, including turrets, will fire longer without overheating. Overheating of guns can be measured by the amount which the circle of your cross-hairs turns red. The more weapon maintenance you have invested in, the slower the red circle fills up, and hence the longer it takes before your guns overheat. On bombers your turrets will shoot longer in manual mode (that is, when you are firing them in gunner view, not AI). AI gunners do not overheat their guns, they just run out of ammo.
When dropping bombs, they tend to deviate from their intended path. The longer the falling time (the higher you drop them from), the more they spread, so they get less accurate. For bombs the reduction in their spread is barely noticeable.
Rockets are inaccurate and here it becomes a noticeable effect. As with bombs, the impact area shrinks with rising skill. However, even with a high skill level, the spread cone stays quite large in radius. Rockets will remain a close proximity weapon.
The advantage of this skill for bomb and rocket accuracy is questionable, simply due the way they are used in combat. Rockets are for close range. The spread increases with altitude, where your cross air becomes bigger relative to the target and the slightest movement means 500m difference on the ground. When flying at tree top level though, the bombs are too low for the skill to apply its usefulness in sufficient time. Aiming bombs is difficult at high altitudes anyway and thus big area strategic targets are preferred. Low altitude bombing is, due the short bomb fall time, accurate anyway. Do not invest in it for these mere purposes. E.g: An Arado 234B does not need this skill.
For anything with guns though and even more so with turrets: Invest. This is an absolutely essential skill for any bomber with at least minor defenses. Only rivalled by experienced gunners and gunner vitality. Shooting in long bursts is essential to keep fighters scared of you, not to mention scoring hits on them, while they attack your plane.
Fighters with weapons prone to overheating benefit too, but in combat firing in short bursts is better anyway, making this skill less relevant. It gains importance though in attacks versus bombers. When a great amount of shells have be fired on a single target. Thus heavy fighters and dedicated single engine fighters may benefit from this skill, too.
Attackers can profit from it as well. Going in guns blazing against a convoy, can end in a dilemma when your guns start to overheat.