If you have 3 stacks of gunpowder and 1 stack of paper in a crafting table and shift click, you get 3 stacks of flight duration 3 rockets. Now if you do the same thing but remove 1 gunpowder from 1 of the stacks, you get the same result except 3 of the rockets are flight duration 2. So if you want to make a lot of flight duration 2 or 3 rockets it can be pretty annoying when you make the wrong kind by accident.