What you want to get out of when you are doing obervational sketches:

build the Visual library / study : We wanna study how things are. The way we study is to look at them, draw them and memorize them, that's how we build our visual library.

Proportion: learn to be aware of the large/media/small design elements in things.

Transition: transitions of surface and form, how it transist from one form(surface) to another. What is an elegant trasition from one to another?

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When you study the thing, you wanna build the visual library, we study it by the mindset that "How this thing work?". Imagine you are an alien and you want to explain these thing to another one, what the things that you would pay attention to? What element make it stand out?