As a beginner to editing my work, I would often apply broad sweeping global edits. Partly it was because I thought that this was the right thing to do. Partly it was because I didn't know any better. Mostly though: if someone had suggested I edit individual local areas of the picture, I wouldn't have known which areas to change or why I would want to change them.
Global edits may feel like the quickest and most obvious way to adjust an image; you get maximum effect for little effort. Want the image to be more punchy? - then just increase contrast across the entire scene and it will definitely feel more exciting to the eye. That's certainly how I felt about global edits when I started out editing my work.
Global contrast applied: Everything is 'hard-toned', and my eye is jumping from the black volcano to the sand in the foreground and then to the hill on the far right-hand-side. My eye is being pulled everywhere.
Image was left 'soft' and I applied careful local contrasts to the Volcano only. This gives the 'impression' that there is contrast in the image, while maintaining softer tones in the frame. Thus resulting in a more 'calm' and less fatiguing image to look at.
However, In my experience, at the initial stages of an editing session, I have rarely found a global edit to be the correct thing to do: often the amount of change that I wish to impart on one area of a scene rarely works with other areas of the scene, unless of course, all the relationships and their proportions to each other are in place. This is rarely the case: often an image starts off with some areas requiring more work than others, or some areas requiring to be quietened down while other areas need to be made louder.
By adding contrast and saturation at the very beginning of your editing session, you can lead your image down the wrong road. Lightroom's ordered panels that suggest a workflow encourage 'baking in' global edits at the very start of your editing session. Something I wouldn't recommend, unless you know you need to brighten / darken an image. But trying to achieve 'final contrasts and luminance here' is a bad approach.
The other thing that I don't like about global edits, is that I may (and often am) affecting areas of the image that I don't understand quite yet, or haven't looked at in greater detail / understanding. I am essentially blind to these areas because when I apply a global edit, I often only notice the areas that I'm interested in changing, and don't notice the areas that I don't want to affect. Rarely do I understand until much later that my global edit has had a negative impact on some area of the picture I wasn't aware of.
Some things can't be undone
There is in my opinion, a lot of bad advice out there. The Lightroom recommended process of walking down the right-hand-side panel in order is, to my mind, prone to error. By trying to achieve final contrasts and luminance here at the very beginning of your editing session encourages baking in tonal adjustments to areas of the picture that may not require them, and will be difficult later to undo them.
Often when contrast is added, it is often difficult (read impossible) to undo it on further adjustments. This is similar to shooting in hard light: you can't take away shadows and contrasts if they were in the original scene, no matter how much contrast reduction you wish to add, but if you start off shooting in soft light, you have the luxury of adding contrast in to suit, and you can do it to varying amounts throughout the frame also.
With this in mind, if we go back to thinking about our RAW editor settings, it makes sense to leave the blacks and whites and contrast at default settings, so that if there are any smooth tonal graduations in the frame : they remain intact.
More contrast means less smooth tonal graduations
This is really the key to this post today: adding contrast as a global edit at the very beginning of image editing will reduce smooth tonal graduations in the frame. You make the image tonally 'hard' and the eye is pulled all over the place - everything will be shouting for the viewers attention.
Conversely, if you take my advice, and deliberately leave your RAW settings so the image is quite soft and flat, you can add in the contrasts and punch to local areas of the frame that need it, while maintaining many of the smooth gradual tonal shifts. The final result will be more restful to the viewer's eye and will also reduce the chance of viewer fatigue.
Global edits are worthwhile
Having said all this, it is worth pointing out that global edits do have their place. For me, they are used to 'equalise' the picture once all my local adjustments have been made. Once I feel that all the adjustments are now in place, but the entire image needs to be either brightened, or darkened, or perhaps some colour cast needs to be removed, this is when I will work with global edits.
So for me, if I were to sum it up:
Local edits are for 'interpretive, creative intentions'.
Global edits are for 'equalising' or 'finishing' a picture.