Back in 2015 a study done in Sweden took non injured male adults and hooked them up to an EMG (electromyography) assessment system.
The test subjects performed arm range of motions in both an upright posture and a slouched posture.
The test subjects who were slouched not only had decreased range of motion but also had increased muscle activity while performing the tests.
What this means is that the more you slouch the worse your range of motion is and also the harder your muscles have to work in order to perform daily tasks.
Proper posture will save you from overusing the neck, back and shoulder muscles and in the long run reduce chronic neck pain, chronic back pain, stiffness, rotator cuff shoulder injuries, headaches, even TMJ problems since they have been linked to forward head posture as well.
Daily chin retraction exercises of 10 per hour while your sitting at your desk will help counter the stress.
To perform these posture exercises try to create a double chin by pulling your chin straight back (not looking up) and lifting your sternum/breastbone to the ceiling. You should feel a pull at the base of your skull as well as into your mid and even lower back.
Watching your back and neck for you