The exact reason for any individual grille cannot be determined from its appearance alone. Some were designed mainly for security, others for decorative effect, and many combine several purposes.
The most interesting feature is how the design balances practicality with craftsmanship. Rather than treating security as something that must look industrial, traditional builders often turned ironwork into an important part of a building's character.
Chapter 2: Creating Space Beyond the Wall
One major advantage of an outward-curving grille is the extra space it creates. A flat grille sits close to the window opening, while a projecting design provides a little more room beyond the wall.