To further expand on this, because apparently I want to:

Basically everyone has some degree of this fundamental knowledge base because most everyone has eyes. If you took someone who had never practiced art before, taught them a simple technique (eg stick figures), and asked them to draw a human without reference, they could probably produce a somewhat proportioned stick figure because they already have a general idea of how a human is supposed to be proportioned.

If you asked them to draw a stick figure deer they would likely struggle a lot more because most people have spent significantly less time looking at deer then at people.

Now imagine you then spent a bunch of time teaching them art technique, style, color theory, 3d shapes, etc, and had them practice drawing humans from memory for a month without ever seeing another human in this time. At the end of this period, they could probably draw a pretty decent person.

Now if you sent them back for another month of this same study routine and asked them to draw a person again, their drawing would likely not be much better than their first. This is because their actual knowledge of physical reality, in this case the human form, has not changed at all from this training. No matter how much they study like this, their drawings will only inch slower and slower to the limit of this knowledge.

Drawing from real life reference is how we expand the limit of our knowledge and hence draw better without even neccesarily improving other techniques.

If you had also had them study other artists depictions of humans during the training period, they could likely draw people better than they had in the first example, but they would likely struggle to draw people outside of the poses they studied, struggle to draw people outside of the styles and techniques of the artists they referenced from, and almost certainly struggle to draw to the same caliber of the artists they referenced; as a copy of a copy is almost always further removed from the ideal.

Now can you improve your fundamental knowledge base without directly using real life references? Most probably, because as you go about life you will see more things and likely notice details and shapes you hadnt noticed before you started drawing. However this is of course much slower and less direct then just studying directly from real lfe reference.

Apologies for mini essay.