When loading your muzzleloading firearm, why is it important to eliminate any air space between the projectile and the powder charge?

When loading muzzle-loading firearm it is important to eliminate any air space between the projectile and the powder charge because it may create dangerous pressure build inside the muzzle.

The high amount of Pressure build-up may backfire the bullet and may cause harm to the fireman. It creates a high magnitude impulse on the fireman which can be dangerous for his body parts. High pressure may also explode the gun powder on the spot and causes a fire.