@commando95 The 3 anatomical parts is just outdated knowledge, in medicine/anatomy we learned that classification and it was correct in the exam but today we know that this was wrong. That would be the only muscle that is classified by it's insertions instead of it's origins.
https://www.ncbi.nlm.nih.gov/pubmed/21118198
The anatomical study demonstrated that the deltoid muscle was divided into seven segments based on the distribution of its intramuscular tendons. The PET study revealed that the intake of FDG was not uniform in the deltoid muscle. The area with high FDG intake corresponded well to the individual muscular segments separated by the intramuscular tendons. We conclude that the deltoid muscle has seven anatomical segments, which seem to represent the functional units of this muscle.
The acromial part is called lateral delts if you look at someone with cannonball delts and low BF you can see that despite having the same origin (all 3 heads have the same insertion) that muscle is not very homogeneous.
https://i.ytimg.com/vi/wMZI-Hn1k3A/maxresdefault.jpg
3 tendons at the insertion but 7 at the origin there are muscle fibers that start at the acromion but end in the tendon of the front and rear delt. That makes the side delts a complicated muscle, just moving it always in the same plane won't be as effective as doing a well rounded training for all parts.
It should be obvious that muscle activation changes when you lean forward or backwards since you change the angle between the force vector and the direction of the muscle from origin to insertion. And it's still the same movement that the muscle performs it's just small tweaks that activate it slightly different at slightly different leanings, which shouldn't be controversial.
If you always do the exact same thing you won't be hitting it as effectively as you could, side delts a weak spot for many people and most of them would benefit from a bit of variation.
Upright rows effectively hit the side delts, even more so if you prefatigue/preactivate them with isolation movements. That alone are 2 different tweaks, they are also activated a bit in front and rear delt exercises. Add in another leaning position for lateral raises and you have a full training of the side delt.
Similar stuff applies to the front and rear delt. You need more than just 1 exercise for them.