It is important for the sake of a stone’s beauty that this angle is of just the right measure. This applies to the center stone on your favorite diamond arrow ring, as well as every other stone that is not riddled with inclusions.
The ideal pavilion angle, according to the biggest number of experts, is 40.75. Most major gem labs agree with each other on slight variations being able to produce well-cut stones. This angle influences the light entering via the table, which then bounces off the opposite pavilion facet, and leaks out via the crown facets.
If the angles of incidence and reflection create a separate angle which is higher than 24.5 degrees, light is not allowed to leak out the bottom of the stone. If the pavilion angle happens to be too steep, the light goes out the opposite side, subsequent to being reflected once on the inside of the stone.
Visually, this manifests as a dark center, which is definitely not desirable. If this is too flat, the light exits right away, because the angle of incidence is inside the critical angle of 24.5 degrees.