Removing What's Not Supposed to Be There
It is said that painting is consists of adding what you want to a blank canvas, whereas photography involves removing what's not supposed to be there. And once you start, there are all sorts of things to consider taking away.
A random photo of some subject will rarely come out that good. Sure, that subject is in there somewhere, but there will probably be other elements that distract the viewer's attention. They're probably not supposed to be there, and removing the competition makes your intended subject stand out better. One of the first rules of good composition is simpler is often better. By cropping and recomposing to remove those elements, you can create a stronger image.
But once you start looking, there are other things entirely that can get in the way, too. Where do you draw the line?
Some problems move with you as your camera pans across the scene. It's uncanny, like the eyes that seem to follow you in that mysterious painting over the fireplace. Get a fingerprint or spot on the front element of your lens, and no degree of in-camera cropping will rid you of that curse. Your best bet is to avoid getting that spot there in the first place. I'm a big fan of lens hoods because they help keep your fingers away from the glass. Some people advocate using a clear filter to protect the front element, but that never made much sense to me. It merely moves the problem to a different glass element, and heaven help you if you get a fingerprint on a glass layer between the filter and lens. It's bound to happen unless you leave that filter screwed on permanently. Unless you're shooting in blowing sand or other harsh environments, you are better off without the added glass element. Remove any spots on your lens, and then do your best to keep it clean. With a bit of care and attention, I find I rarely need to re-clean my lenses in the field.
Dust spots on your sensor create a similar but even more troublesome problem. At least a smudge on your lens will go away when you change lenses. I've yet to see a camera with interchangeable sensors. But as with lens spots, your best strategy for dealing with dust on your sensor is to avoid the problem in the first place. Sooner or later, you will need to clean your sensor, but the less frequent, the better. Most camera stores will clean your sensor for a fee, but I'd suggest learning to do the chore yourself if you can. Some photographers are too nervous about venturing near the sensor, but the job isn't difficult once you know how to tackle it.
But dust spots aren't supposed to be there and need to go one way or another. If you miss them before pressing the shutter release, you'll have to deal with them after. Thankfully, Photoshop, Lightroom, and every other digital editing application have tools specifically for this chore. While I'd prefer not to spend my time cloning out dust spots, it's sometimes necessary.
Those same digital darkroom tools can remove other distractions, too. When shooting, I try to keep an eye out for the unfortunate residue of modern commercial civilization. You can find chewing gum wrappers and cigarette buts everywhere. Some people have even discovered iPhones and car keys that others lost. Someday, someone will come across your lens cap that the wind whisked away years back now. I've left a few myself over cliffs and down ravines. Even with the best of intentions, it's possible to leave things behind that don't belong there. Trash has a simple solution. I can carry it out and clean up after others. The disposition of a found smartphone would be more complicated to resolve. But if I didn't catch something while on location, at least I can get rid of it on my computer.
Then there are the coke cans my camera will unavoidably see that I have little chance to extricate physically — those used to really bug me back in the slide film era. Things are a lot easier these days.
There aren't any limits other than time and skill to what you can remove in Lightroom. Even more so in Photoshop. If you decide something is not supposed to be there, and you're up to tackling its removal, Adobe and others are only too eager to assist. You can even remove the whole sky now if you prefer a different one. I've had to wait for others to get out of the way when composing a shot many times. I suppose I could clone them out later if I wanted to, but I prefer not to go that far. Instead, I patiently wait for them to remove themselves. It doesn't always work, but I have to draw the line somewhere. And where that line is varies based on what I plan to do with an image, too. It depends.
So I guess my point here is that it's up to you. Years ago, there were more limits to what was possible, but the advent of digital made most of them as obsolete as it did the rolls of film I used to keep in my refrigerator. And what limits that remained have been dropping. And on both the software and hardware dimensions, the pace of progress has been accelerating.
So, is something supposed to be there because it is there, or because it fits your expectations for what should be there? At what point is it OK to remove it? I've already confessed to some things on both sides of my line. But where do you draw yours? That's not something I can answer for you here, and Adobe and Intel won't tell you either.
But I believe every working photographer, both amateur and professional, should put some thought into it.