Lots of kids see the answer directly for the simple problems, and so don't see the point of showing their work.
The point of showing the work is to learn the mechanics of solving the problem. If you do not learn the mechanics of solving for simple problems, you will not be able to apply the mechanics to more complex problems where one can no longer intuitively see the answer.
The point of showing the work is to learn the mechanics of solving the problem. If you do not learn the mechanics of solving for simple problems, you will not be able to apply the mechanics to more complex problems where one can no longer intuitively see the answer.