what is the purpose of power brake booster?

This can be normal to a certain level. Most cars with a brake booster will give you a puff type sound when you hit the brake. If it's become much louder than normal this could be an issue with the brake booster under the hood. You would want to make sure there isn't vacuum leaking out of the booster. If you don't normally work on your car then use this link to find a good shop near you. https://repairpal.com/repair-shops Alex

The booster is what gives you power brakes. Without it your pedal would be very hard to press.