One ring is nice. A stack of rings is twice as nice. Not only does it look great, it also allows to change up the look of your set as the mood strikes you.
Do you have fingers that swell a little when it is warm outside? Do you need "summer rings" and "winter rings" because your size changes so much? If that is the case, then stackable rings are a great way to solve the problem. Just switch up the amount of rings you wear and the stack will feel a bit tighter as you are adding more rings, and looser as you wear fewer rings.
And of course, in any case, it is easy and fun to change the combination of rings for a different look.