There's a big difference between seeing and observing. Seeing allows you to recognize, however observing allows you to understand reality.
That's why practicing proportions is so important. When you're looking at an image and analyzing how items are related to one another, you get a sense of how that object is built up. You get a sense of how things fit together. You get a sense of proportions.
This ability is what helps us make a image especially a face look real.
I still struggle with this, but I'm happy to say there have been some big improvements over the years.
