Cat nets are used to provide a safe living space by way of containment. Cat enclosures can be detached/standalone or simply be used to net in a preexisting semi contained area such as a pergola.
There are very valid reasons for constructing net enclosures - Council bylaws often require containment of domestic cats between certain hrs of the day for the protection of wildlife. Cat netting, whether in Perth, or throughout Australia, also offers physical protection from other animals including snakes, dogs, or even other cats. If you have a wanderer there is also the possibility of theft or vehicle impact.