Norodnik, it aids wing bending relief.
If ALL the weight of the a/c ie fuel, payload & airframe was kept in the fuselage then there will be a large amount of stress on the wing root (Lift from the wings trying to make them go up, Gravity trying to make the fuselage go down. Where they meet has a conflict of ideas...)
By burning what fuel there is in the fuselage first, and keeping as much as possible stored in the wings, SOME of the stress on the wing root is reduced.