Not sure if anyone really answered this... I'll attempt.
First, let's be clear, Brinks Home Security became Broadview Security in June 2009, and was then acquired by ADT in 2010. So, Brinks no longer exists.
Companies like ADT, FrontPoint, and Vivint offer both the equipment to secure your home, and also the monitoring services. Where these companies make their money is on the monitoring, not so much the equipment. I'll explain:
The average monitoring contract is 36-42 months. Depending on the company, this can range from $700-$2900 in total cost over the lifetime of the contract. Now, take into consideration the fact that a company like ADT has an average customer retention of 8-10 years... you get the idea? already said this below, but I wanted to provide a bit more detail.
Now that we understand why the monitoring fees are there, let's consider what you're actually getting:
It's true what Bryan said, that even un-monitored alarm systems have systems in place to send text-message alerts when the alarm is triggered, but that puts the responsibility on you, the consumer, to monitor your own alarm. If you're busy, away from your phone, in a meeting, etc, you won't be able to respond fast enough to do anything about the alarm that's blaring at your unoccupied home. Really, who wants to be on constant alert? What if you are hiking with no reception? Doing Yoga?
This is where the professional alarm monitoring services comes in. What you're really getting is a team of people sitting ready to respond within seconds of your alarm being triggered. Those agents then notify the authorities through proprietary channels to ensure a minimun response time.
ADT, for example, has a popular system called 2-Way Voice, in which the moment your alarm is triggered, an agent get's on a built-in intercom and checks to see if the alarm is indeed a real home intrusion, or a false alarm. If the homeowner or family member is home, they simply speak the password they've set and the alarm is dismissed. If the agent get's no response, then the authorities are notified.