Here read something about it by the time you're finish you'll be an expert on what it does.
The Body Control Module (BCM) supplies vehicle occupants with visual and audible information and controls various vehicle functions, To provide and receive information, the module is interfaced to the vehicle's serial bus communications network, This network consists of the powertrain control module (PCM), the Transmission Control Module (TCM), the electro/mechanical instrument cluster (MIC), the Airbag Control Module (ACM), the Compass/mini-trip module and the Controller Antilock brake Module (CAB). The BCM is operational when battery power is supplied to the module. Ignition switch power is needed for ignition switch functions.
The BCM provides the following features: Automatic door locks
BCM diagnostic support
Door lock inhibit
Headlamp time delay
Ignition key lamp
Instrument panel dimming
Mechanical instrument cluster support
Power door locks (with/without remote keyless entry)
Sliding door memory lock
Vehicle Theft Security system (VTSS)
Windshield wipers / washers (front and rear)
As you can see, it is central to the control systems and can be very complex. To my way of thinking if one or maybe two systems connected to the BCM have a problem, then the cause is probably within that system. But when they all have a problem, then I have to think the BCM is bad.