For the same reason kids learn to count with blocks instead of digits. Visual learning is it's own channel, apart from audio or kinetic learning or even text processing. By drawing a diagram, you're engaging visual and kinetic learning systems in your brain.

For me it's due to the feedback loop: head instructs the hands, hands draw the diagrams, eyes read the diagram back in, head reprocessed the diagram, rinse and repeat.

It's also why writing revision material out in long hand helped it persist in my memory back when I last studied

Drawing is thinking. Organizational thinking. Viewing a problem from a different perspective.