Working by hand is a skill which involves using the creative side of the brain. Working by hand enables this to happen through a physical connection between our brains, eyes and hands. The connection between our brain (thinking) and hands (mark making) is what brings our creative side of the brain into the equation. Working by hand helps us get our ideas out of our heads and onto paper, lets us explore our work, explore the potential possibilities. Putting ideas onto paper helps us to develop our ideas further, allows us to take risks with our own work and possibly come up with a final solution or idea.

Above is one piece of work, of many, created by Leonardo. This exact piece of work is the invention of some weapons. You can clearly see the developments he has made from just drawing a few of ideas onto paper. At the top of the page you can see the ideas he has come up with and developments he has made from just drawing his first idea on the paper. At the bottom of the page you can see what his ideas from the top of page have developed into, looks like a crossbow. Around that idea you can see other intricate details which would help his device work, he hasn't just drew one idea for the little details either, he has drew a couple ideas and explored which idea will work and which idea that won't work.
From this piece of work you can also tell Leonardo was
using his other side of the brain too. Next to his idea at the bottom he has written notes. This is just text explaining his ideas and explaining the development his work went through. He is evaluating his idea, what could work well, what might not work well and how he can improve that idea.
I believe from looking at this piece of work Leonardo was both drawing to explore his ideas and drawing to ask himself questions about his ideas. I think he was drawing to explore because he wants to see where his ideas could go, how they could develop, he’s exploring his ideas to see how they might develop, exploring the possible ways how his ideas could work.
Leonardo is drawing to ask himself questions because, In his head his ideas might seem that they work, where as drawing his ideas on paper will answer his questions whether his idea would work or not, by drawing his idea down he has able to draw and explore other ways to see how his idea might work. In his head he could have asked himself the questions, how could my idea be improved? How might I be able to make my idea better? How can I make my idea work? There is only one way he could answer these questions and that is by working by hand, seeing his idea through his eyes and hands to get a better image of what exactly he is thinking.