Why do you is it important to slowly add NaOH to CuSO4?

yes, it forms a blue gelatininous precipitate if copper (II) hydroxide. It will form whether you add it quickly or slowly, so I guess it was so you could actally see the precipitate forming as each drop of NaOH solution was added and the blue ppt forming, until eventually you get blue pppt at the bottom and clear liquid above that. Pour it in quickly and it looks like blue soup!