It is one thing to copy someone else's drawing.
Another thing to understand the drawing you are copying.
And yet another thing to be able to draw what you've copied from memory.
The same thing applies to observational drawings.
It is one thing to draw what's in front of you.
Another thing to understand what's in front of you when you are drawing.
And another thing to be able to apply the knowledge gained from the observational drawing to your imaginary drawings.
The last task is the hardest, and therefore makes it a good standard in terms of judging one's drawing skills.