Why is it very dangerous for the driver and
passengers if any bubbles of gas get into
the brake fluid? Even worse what would go wrong if the break fluid leaked out of its pipes?

If it leaks

If there is a brake fluid leak and air gets mixed with the fluid in the brake lines, then the fluid will not be able to flow correctly. As a result, when you press your foot down on the brake pedal, it will feel squishy or spongy. In other words, the brake pedal will feel very soft.

If air bubbles get into

Air is the enemy in a hydraulic system because it reduces pressure and essentially makes the process less effective. When air gets into the brake lines, your brake system suffers. Symptoms that can indicate you have air in your brake lines include the following: Brake pedal feels spongy when you press down

Hope it helps