In very brief: BMC's (Bleed Monitoring Computers) provide automatic engine bleed air control; air is taken from Engine IP valve (from HP to meet high demands), APU or an external source. Bleed air is sent to packs, cooled down (ACM, heat exchangers) and supplied for pressurization, air conditioning, avionics ventilation, water tank pressurization, hydraulic reservoir pressurization, anti-ice and engine start ... without packs, compressor air would be too hot for being delivered to the users (150-200° C) ... that's it !
