Drawing is the probity of art - Jean Dominique Ingres. I had to look up probity: uncompromising adherence to the highest principles and ideals. That could have more than one meaning.
One is, you can't fake it in drawing. In painting, you can dress things up with color, texture, showie brushwork, etc. But in drawing its just you, your pencil and the paper. You are somewhat naked to the world showing all your strengths and your faults.
This is good in at least 2 ways. First, it reveals what you need to work on. But more importantly, it gives you the most direct form of expressing your ideas and perceptions.
Milton Glaser tells a wonderful story.* His wife brought home a gift, a new turntable for his classical records. The store owner said it was the best you could buy. It was just a slab of wood with a turn table and the stylus arm. No automatic retraction of the arm, no dials or controls except for an on and off switch.
"What did you pay for this"?
"A thousand dollars".
Milton went back to the store and asked the owner what was up with this simple device costing $1000?
"Did you swindle my wife?"
"Did you try playing it?"
"Go try it and then come back."
Of course, when Glaser played it, his records sounded better than ever. When he went back to the store, the owner explained that for the best sound, you want as little interference between the initial signal and the final sound. Every time you add a spring for automatic retraction of the arm, a dial for this, a knob for that, you create interference.
In drawing, its just pencil and paper, dark and light. Straight from your perception to the final image with minimal interference. Probity.
*I can't find the original story so this is best as I can remember it. Details may be off but the basic idea is accurate.