The AC compressor bracket bolts the A/C compressor directly to the engine block. Because the accessory belt pulls hard on the compressor pulley, this bracket must be extremely strong. It is usually made from thick cast iron or structural aluminum.
If the mounting bolts work loose, the bracket will vibrate. This vibration causes belt noise and can snap the bracket or damage the engine block threads. Keeping these bolts torqued to spec prevents accessory drive failures.
When replacing a compressor, inspect the bracket for cracks or corrosion. Aluminum brackets can corrode in salty climates. This buildup can push the compressor out of alignment, causing the belt to walk off the pulley.