My understanding (IE, I may be incorrect on some details) is that due to he extremely long barrel length of 7'11" long, the powder used is extremely slow burning. Using only the slow burning smokeless powder charge alone may result in incomplete or inconsistent powder conflagration, which at best results in lag time, but at worst could potentially result in a pressure spike, which in a gun producing close to 35,000 foot pounds of energy at the muzzle, is undesirable.
To ensure an even burn of the powder, a small charge of fast burning black powder is contained just over the primer in a teabag to assist in even and complete conflagration of the main powder charge.