Lets say I am a farmer and I take eleven pigs to market. I start with XI pigs and sell three III. I know X = VV and I know V = IIIII so I expand XI = VVI = V IIIII I, then take way III sticks and am left with VIII. It is easier for simple primitive thing because the stick I represents a thing and the V represents 5 I's and X represents 2 V's. It is perhaps easier for someone like a farmer in Roman times to use than remembering arithmetic rules. It is more directly related to the physical world. At least that was my hypothesis.