I teach my classes from a drawing "science" perspective. That is, while many think of drawing as just an art, I think of drawing as a science and art.
To better explain my point of view, we must look at another discipline: writing. The rules of English spelling and grammar are the same for each person. Without it, no one would be able to understand each other's writing or speaking. That is science. There is right and wrong.
In writing, the art comes from how you use spelling and grammar. It's the difference between a grocery list, this blog, and a Charles Bukowski story.
In drawing, how you use the science of drawing is what determines the art. But to do art, as in writing, you must first learn the science. Learning the science doesn't force you to use it each time. But it does give you the option for using it when you do need it.
In my cartooning classes, I teach this science by having students draw toys. That way they can check their work against something real and get a taste for the fact that good cartoons have an element of solidity...a sense that the cartoon is real and can be held and touched.
It's also a practice suggested by one of my favorite artists, the creator of the Ren and Stimpy.
The point is to draw what you see as exactly as possible and correct where you mess up. Observation drawing is a test and opportunity to improve your hand-eye coordination, without which it is far more difficult to invent and draw from your imagination.
After all, if you can't draw what is in front of you, how can you draw what you only see in your head for the briefest of seconds?