Michelangelo used his drawings to prepare his paintings. His drawings were merely sketches used to help him through whatever painting or sculpture he was working on at the time.
He used them to find poses and capture how the people in his art would stand, sit, or otherwise. It seems as though Michelangelo’s drawings were an emotional part of the process as well. He did not like showing them to people maybe because he did not want people to see that side of his masterpieces; he wanted them to only see the beautiful finished product.
Da Vinci was a scientist as well as an artist. Many of his drawings are of the human body and skeletons. It doesn’t really look like he used his drawings in the same way as Michelangelo. Da Vinci’s drawings seem to be more accurate and detailed than Michelangelo’s. To me it seems that da Vinci’s drawings were more of the scientific side of him. They were detailed and labeled and I am guessing they were used as study tools.