For general cctv, many are installed to allow monitoring while away from the house. Nanny cams for example.

Installing cctv to an existing network will put it online automatically.

The most important issue to consider is these devices - routers, cameras, alarms, locks... come with default passwords. And almost no one changes them. So anyone who knows which port (Shodan search engine, port scanner) to look has a high chance of getting entry.