It should be quite obvious: drawing requires constant visual feedback. Why is it then that we still draw with code in so many situations? Of course it’s because the low-level APIs always come first, and design tools are built after and on top of those. Existing design tools also don’t typically include complex UI elements such as buttons.
When we launched our Touch Display module for Netduino Go!, we naturally built APIs that made it easy to draw on the screen from code, but very soon, we felt the limitations and tedium of drawing in code. In particular, any modification requires a modification of the code, followed by compilation and deployment. When trying to set-up buttons at pixel precision, the process is not optimal. This is why we built Nutshell.