To answer this question of "In human anatomy what is the purpose of measuring shoulder abduction and shoulder adduction separately if they cover the same range of motion?"
Before looking at some example images of video clips showing movement I think it's easier to consider wrist flexion/extension, and knee flexion/extension, as the concepts are very analogous.
In the case if wrist flexion and wrist extension, 0 is neutral. And the wrist can move maybe 70 degrees either way.
In the case of the knee, 0 degrees is full extension. Some people have a few degrees of hyperextension. And basically the movement people do in a gym is moving through degrees of knee flexion, on a machine known as a knee extension machine.
So if we apply that to the case of the wrist, when people say that wrist extension typically has a ROM/range of motion of 70 degrees, they mean when measuring from 0. A person could do wrist extension from a position of wrist flexion, and do e.g. 140 degrees of wrist extension.
So, degrees of extension and degrees of flexion do not overlap.
But the terms extension and flexion do not assume/necessitate starting from a particular position of extension or flexion. They could start from any position of extension or flexion or neutral.
In the context of measurement with a goniometer, and measuring a person's range of motion, then it suggests starting from 0/neutral. But otherwise, just talking of the movements of extension and flexion, it doesn't require starting from zero.
So to answer the question of "In human anatomy what is the purpose of measuring shoulder abduction and shoulder adduction separately if they cover the same range of motion?"
If each starts from zero, then they don't cover the same range of motion.
If measuring a person's ROM/range of motion, as physiotherapists do, then one would start from zero.
If not measuring a person's range of motion, e.g. if just interested in the movement then the movements can cover the same range. One might extend from a position of extension. One might extend from a position of flexion, and from there either stay in flexion, or go into extension. Likewise one might flex from a position of flexion or extension or neutral/0.
Like forearm supination can start from a degree of pronation and finish at a degree of supination, or start from a degree of supination and finish at a degree of supination, or start from a degree of pronation and finish in a degree of pronation. A "long" forearm supination movement not measured from 0, would start from a high degree of pronation and finish at a degree of supination.
I think the term arm abduction/adduction normally refers to vertical abduction/adduction, but i'll mention horizontal abduction/adduction too.
A screenshot from this video on youtube, man with red dot on his shoulder, shows the furthest degree of (vertical) shoulder adduction.
That's consistent with what I heard from somebody that told me "normal (vertical) shoulder adduction motion is 30 to 50 degrees. Normal range of motion for shoulder (vertical) abduction is 150 degrees. "
There is an image from https://www.anatomyexhibits.com/ showing 0-180 degrees of vertical shoulder abduction.
Without reproducing it here , it's like
And they have an image showing 0-30 degrees of shoulder vertical adduction
which you can imagine with a protractor showing 0-30 degrees beyond 0. So the arm starts by the side at 0 (neutral position), then moves across the body like the first pic(the man with the red dot on his shoulder).
The full movement of adduction would go from a high degree of abduction, down to 0 degrees of abduction / 0 degrees of adduction, and through to a high degree of adduction(as far as the arm goes for adduction to the end of the movement).
So likewise, with the hips , degrees of hip adduction are different to degrees of hip abduction. Though the movements of abduction or adduction can pass through degrees of one or the other or both. And that applies with wrist extension and flexion too. But for measuring a person's ROM you'd start from 0.
And I notice that the anatomyexhibits link specifies, vertical shoulder abduction/adduction. Horizontal shoulder Abduction/adduction would be in the transverse plane (I have noticed a difference though where some eg anatomyexhibits, measure horizontal shoulder abduction/adduction with 0 out in front, whereas others measure it with 0 to the side).
e.g. this link https://www.researchgate.net/figure/Shoulder-horizontal-adduction-A-start-position-B-end-position_fig1_309743960
has horizontal shoulder adduction (or no doubt abduction), with 0 out to the side.
whereas anatomyexhibits.com has 0 out in the front
This 7sec video saying it shows shoulder adduction.. youtube.com/watch?v=2N6WhuGDZRs is showing it not from 0. it's showing it from 180 or nearly 180 degrees of shoulder abduction.
One strange exception I ran into is 43sec into Goniometry Shoulder Abduction & Adduction (by kelly) youtube.com/watch?v=WgTu6HsZZTE it shows adduction measured where 0 is arm up. That seems strange to me, normally abduction and adduction are measured from the same 0 point.
This video by OTStudentVids shows Horizontal shoulder abduction and horizontal shoulder adduction and takes 0 as out to the side. https://www.youtube.com/watch?v=Zu0WPhfHCWc All makes sense, abduction is one way, and adduction is towards/ across the body.