Why don't car brakes work if air get into the pipes?

Air in the brake lines can create a spongy feel in the brake pedal and reduce braking efficiency because air is compressible, while brake fluid is not. When the brake pedal is pressed, the air compresses instead of transmitting the force effectively to the brake components. This can lead to a significant decrease in braking power, making it difficult to stop the vehicle safely. To restore proper function, the air must be bled from the braking system, ensuring that only brake fluid is present.