The mid back, or thoracic spine, needs frequent movement to prevent stiffness, and possibly reduce the amount of pain and discomfort you feel on a daily basis. It also needs to have significant mobility to maintain a healthy and pain-free neck and shoulders.
And although this post uses the word "stretches" they are really mobilization drills. These are ways to get the joints to move through greater ranges of motion, not by necessarily stretching the muscles around the joints.
Frequently people fall into the trap of doing a few cats and camels, or even branch out and do a cobra or two and call it a day. Although those drill shave a time and a place, adding a little bit of load via a kettlebell or dumbbell can have a transformative impact on your body and the way you feel.
All of these drills will add some load through the mobilization drill, allowing for greater stimulation of sensory receptors in the body, and giving your nervous system the needed input it needs to truly gain mobility. And of course you do not need to a kettlebell, you can use a dumbbell or small weighted ball if you want. Hell, you can use these are regular mobilizations without weight, but I assure you, you will get a little more out of these even if you use one pound.