If a trader is long one contract with brackets (SL and TP), and then decides to add to that position, the new contract will increase the quantity of the position, and there will be a line representing the average price of the two contracts with a +2 next to it. However, the brackets associated with that position will not update. If the bracket quantities updated with the position size, then one could start small, and then add more contracts without losing the benefit of using brackets. They could also start large and manually sell individual contracts as the market moved higher, without needing to close the entire position.
With brackets on, adding to the position would add an additional set of brackets for each position. After adding two contracts separately, there would be a total of six extra orders on the screen. It would be far less complicated to just update the brackets to match the position size.