Stacking rings are more comfortable than solid bands of the same width because the rings are free to move on one another slightly while being worn.
This movement lessens pinching and the development of callouses.
Stacking rings do not trap moisture next to the skin.
Many of us, myself included, have enlarged or swollen knuckles. I put my rings on one at a time which reduces the friction and pressure going over my knuckles.
This method also allows me to wear a set that best fits my finger.
Most of us shrink in the winter and swell in the summer which makes our rings feel loose or tight. An outside ring of a stack can be a half size smaller to serve as a keeper ring in the winter. In summer, this ring can be worn next to the palm where the finger is the most narrow.
People who are enthusiastic about stacking rings will frequently add more rings to their original set so they can vary the patterns, color and width.