Explain different type of fire detector?
Two alarm systems don't make for twice the protection

What you have here is a smoke/heat combination detector for a central station fire alarm system.  Note that it cannot alarm by itself -- it requires the "brain" of a compatible central station fire alarm control panel as well as a notification appliance such as a horn or a strobe connected to the panel in order for the system to work and sound the alarm when smoke or excessive heat is detected.

So, in your case, this smoke detector is the detector for your alarm system.  The rest of the hardwired alarms will set each other off when they detect smoke, but only this detector will alert the alarm system when it detects smoke, provoking the alarm system into dialing the monitoring service as well as setting off whatever horns, strobes, or whatnot that have been connected to the alarm system.

In general, buildings only use one type of fire alarm system (multiple-station or central station), not the mix of both you see in your house.  Multiple-station systems are easier and cheaper to maintain and also more familiar to home owners; however, a properly configured central station system can automatically notify the FD if there's a fire, and can also supervise the fire alarm wiring so that the alarm panel will tell you there's trouble if a wire breaks or alert the monitoring company that someone is tampering with your house's fire protection systems.  A properly installed central station fire alarm system with monitoring also may give you a discount on homeowner's insurance; however, monitoring itself is a monthly expense, and central station systems can be more costly and difficult for an individual homeowner to maintain.