The pebbles you see on top of a succulent arrangement are called top dressing.
Top dressing adds a “finishing touch” to an arrangement, making it looked polished and clean. It’s not only aesthetically pleasing, but adding it to the top of your soil also prevents the arrangement from shifting as much over time.
These little rocks also keeps the soil in place when watering, and can also reduce the amount of dust that flies up when you water! That means that more water gets down to your succulent’s roots, and drains out quickly.
Sometimes this can prevent the soil from drying out as quickly, but if you’re using a well-draining soil it shouldn’t be an issue.