Thoughts about Sketching and Drawing
I prefer to think of sketching as an art-form that is different from drawing and painting. It is much like the art of writing, except that one produces images rather than text. Sketching is driven from the soul of the artist and is usually completed in seconds, minutes and sometimes a few hours: but rarely over days, weeks or months. The images are manifested from the emotional perception during the execution and translation of the vision onto the artist’s page, but most important as the French architect, Eugene Viollet-le-Duc, wrote: Drawing …is the best means to develop the mind and form judgment, because in this way one learns to see and to see is to know.
Sketchingjourney's Blog 
Sketches by le Corbusier seems like an “instant message” to convey concepts to himself or a third party, while sketches by Herman Hertzberger appear to be analytical and detailed.
There are many reasons for sketching. There are also techniques for/in sketching too, which can be technical and or mechanical. The act of perceiving and executing a sketch is interesting. Why do we sketch what we sketch? (Besides the obvious)
Why do we sketch what we sketch?
I prefer to sketch what I see rather than “doodle” aimlessly. I sketch to keep my mind active and get away from being bored. Sketching keeps me alert and sharpens my imagination.