Safety and appearance are important factor, but it does also reduce the shear stress concentration at the tip of the stiffener by making the stiffener less stiff at the tip. Stresses concentrate where there are sudden changes in stiffness. It does make the structure stronger. It may not be necessary to make the structure work in any given situation, but it does improve the strength of the structure, and it looks better. No calculations are needed to know that, so the design feature is often a default choice, even though it may add a little cost.
