Lot's people will say the inflation mechanism or that it looks cool but I'm afraid their all mistaken.
When selecting a BCD you'll check out the sizing options for both your height and your weight. The BCD's primary function is to make sure you float and then secure all of the equipment.
If the number one rule of SCUBA diving is to always breathe and never hold your breath. Rule number two is always be positively buoyant at the surface!