Protecting your house requires multiple layers of technology, what is sometimes refered to in the industy as defense in depth. Your first line of defense are effective locks. An alarm system will alert you, your neigbors, and hopefully your alarm monitoring company if someone breaks into your house through an unlocked window or door, or if they managed to defeat your locks. Security cameras serve both as a deterent, they are usful in remotely evaluating alarms and expiditing a response when an incident is really in progress, and they help identify suspects if the security of your house has been compromised.