It's to improve the shootability of a gun. If you can make the trigger pull lighter (within reason that is) and more crisp (i.e. remove the trigger creep), then the shooter should be able to shoot the gun better. It's more important with rifles and pistols, but it's nice to have on a shotgun as well.
Some guns come from the factory with triggers that are hard to pull. When shooting these guns, the shooter pulls the gun off target due to the amount of strength required to pull the trigger.
A trigger job is done only by someone that knows what they are doing. It is a procedure that reduces the amount of strength required to make the gun fire. Usually by polishing the parts in the trigger assembly or changing a spring or two.
After the trigger job, it doesn't take as much weight on the trigger to fire the gun. I like about 4-5 lbs. My Nova came with a 9 lb trigger. Some guns, especially cheaper jobs, come from the factory with 9-12 lbs trigger pull. They could use the services of a qualified-repeat-qualified-gunsmith. Too light a pull is dangerous. Too heavy is conducive to bad shooting.
Autoloaders are not known for having great triggers. My wife's Browning Lady Gold SC is no exception. She likes it a lot but it had a pretty crappy trigger when we bought it but she didn't want me to change anything.
3 months later she was shooting a round of SC with her sister and they exchanged guns at one station. Her sister shoots a 391 parallel target RL. Upon the first target, the 391 went off a little early because she's so used to pulling the creep out of that crappy Gold trigger.... she let out a little yelp. I was kinda funny.
Anyway, enough of the Gold trigger; I didn't want her to get too used to that. I pulled out the trigger group and sent it to Anderson Custom Shotguns for a trigger job. Two weeks later I slipped the trigger group back in her gun without her knowing. What a difference!... very crisp.
I told her later about getting the work done. She thought something was different but she couldn't figure out what. Anyway, she's happy.
The trigger has to overcome three items.
1. Initial spring tension that the hammer/stricker & trigger are under.
2. Dragging of the trigger along the sear: This distance is called creep.
3. The overtravel: Which is the continued backward movement of the trigger once it has released the hammer/sear.
A gunsmith working on some or all of the above, Along with pull or release weight. Is commonly called a trigger job.
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