On guns intended for target use I sometimes upgrade the sights. Because I like big blocky black sights I can see. I mount a partridge front blade and as big a rear sight blade I can find. Though these days it's not hard to just buy them that way.
Trigger job, for obvious reasons, 3-4# pull. Guns these days have much worse trigger pulls than they used too. No doubt because they are just not hand-fitted like they used to be. My Springfield "loaded" 1911 had a horrible (to me) trigger pull. It's off getting one now.
If they make (or made) a Pachmayr grip for the gun I find a set and mount those. Have to go to eBay sometimes for those. At one time or another they've made them for just about everything.
I want some nice wood grips, but I just can't make up my mind.
I want to upgrade my sights on my AR, but again can't make up my mind.
I don't even have a sling on my hunting rifle because I can't make up my mind.
I am starting to get worried about my possible (Low-T) But I just don't know.....
Polish friction points on all new pistols. mess with springs to get a decent trigger. fiber optic front post.
I strongly agree with Glen Wolde, most new guns have horrible triggers compared to 50+ year old guns. Might be the hand fitting skill of the CRAFTSMEN that finished them, or the simplified design or simply the gajillion rounds fired work machined them. polishing new gun guts wisely, helps eliminate much of the gritty feel.
The why is simple, to make all my tools perform essentially the same to allow easy transition from one to another. And a smooth clean trigger transforms a mediocre firearm into a joy to shoot.
I've done grips, springs, ejectors, sights and stuff on revolvers and semi autos. I did an action and trigger job on my r92 lever .357. My shotguns are all stock. Built an AR from an 80% lower and machined out the receiver myself if that counts.
Depends on the gun, every one gets a little different work if anything changes at all. My major one is my RIA 1911; grips, grip safety, hammer, trigger, sights. Only the sights were done by a gunsmith, only because I don't have staking tools for the front sight. All the other handguns are stock, felt no need to change them.
Rifle, pistol, shotgun - this is broad territory.
One usually buys a gun to closely fit their needs.
Many of the 'personal fine tuning' alterations have been mentioned.
Like a performance car - drive it hard and to the best of your abilities UNTIL you know what you need to alter and why.
I bought a S&W99 in .40 just to do a grip chop on it. It was a Police trade in so it has Trijicon night sights on it. By chopping the grip I get a full size slide and sight radius on a compact grip. It still holds 8rds of .40, which is nice.