I have been using pen and ink a lot more lately (and am growing to really like it), but there are still things I find difficult, especially when it comes to executing certain very precise ink ‘styles’ required for scientific illustration (loose pen and ink work is a whole ‘nother board game!!). I think that’s where copying comes in handy. Taking an image in the style you are wanting to learn, enlarging it (so the lines are approximate to the original inked size) and then taping acetate film over the photocopy and copying the original drawing is a great learning exercise. I have found that trying to get all the nuances of the line and details the same as the original illustrator did in their illustration is a huge help in learning the muscle-memory required to execute the technique (or something similar) on your own work.
Here is an illustration of a Marginella pygmaea shell from Henry Suters ‘Manual of the New Zealand Mollusca‘. The original illustration is much reduced in the published work, so that the stippling is hardly visible (and the dots tend to merge).