I'll agree and disagree with some of the comments to date...
First, the tip weight is part of the structural unit called the model. Its basic function is to balance other things. The inboard wing flies slower than the outboard, gradually going faster from inboard tip to outboard tip. At some point betweeen tips, lift to each side is equal. Unequal panel spans - outer panel shorter - is one way to tackle that. "Excess" tipweight outboard - to meet the lift difference - is another.
The weight of the flying lines is not centered on the model, either. The part of that weight carried by the model is all on the lower-lifting panel side. Again, tipweight helps balance that..
These are useful to prevent lift from rolling the model about (around) that heavy cluster of stuff called the fuselage/engine/tail, etc.
Second, as mentioned, we can identify a moment about the cg. Since the wing and other pieces are a one-piece, rigid structure, there is only one center of mass. It may not lie on the principal axes of the structure, exactly, but should be very close to them.
A moment is force acting at a distance from a real or 'virtual' pivot point. Sayyy, whaa??? Examples: Lift is a force; the point across the span where lift is equal on both sides can be considered the point where it all acts. If force through that point goes through the cg, the cg moves in the direction of the force. If it doesn't go through the cg, a moment is created trying to roll the model in response to the greater force on one side.
For a model in flight, the cg (Center of Mass, actually) is the equivalent of the pivot point. Balancing the model about (on) the fuselage long axis for the lift, drag, thrust, etc., conditions reduces or eliminates the tendency of a temporary moment to cause unwanted roll. "Hinging" is a roll/yaw reaction caused by severe moments appearing at high-g maneuvering when the total forces and the Center of Mass pivot don't "agree."
Third, as mentioned, the longer the arm between the weight and the pivot point, the greater the "moment" a given weight provides. But, adding tailweight doesn't really cause a dynamic moment. The weight just becomes a part of the rigid structure. It DOES adjust the Center of Mass of the rigid structure... Which is what you want to do...
So, the least added weight that does the job will be as far from the (incorrect) cg as possible, and as near the line of the dynamic center of forces as possible. (Dynamic - since it is the changes in lift -and its drag- that can cause problems, static (non-moving) solutions just might not be enough.) Weight added to the left or right of the fuselage axis can create an unwanted "moment" in high-g conditions.
In fewest words: ballast in the tail on the fuselage centerline.
IMHO...