I have been lucky enough to see a few oil studies painted by academic painters in some great museums. But these are a drop in the paint bucket in the amount of studies that most great artists produced. Inspired, I took a step forward in my studies and decided to complete a canvas of oil studies.
What I learned as I looked at my hand and painted was that accuracy and practice go hand in hand (no pun intended). The more I paint, the more I practice and the more accurate I get to creating a solid study. These sheets serve the purpose of being studies for large paintings without further reference to the model. Because the model won't always be available to study. So academic artist must be accurate with the brush and be able to "copy and paste" to paint the same hand when it matters in a larger, more finished work.
It really is like being able to manipulate an image through photoshop or another photo editor, but the bonus of doing it by hand, is the fact that it's more time spent with a brush in my hand. I feel like there truly is no error in a great academic painting. I'm going to let the errors happen in the studies and sketches, but everything has to be rendered for a reason in the finished piece. This is kind of a scary notion, to be a human creator of images over and over again. But I try to remember, no one is ever going to care about these oil studies in the future, much of the audience will only see a finished piece. But when I put brush to canvas for the final painting, I don't want any chance that I didn't think of something beforehand. Plus, it's very satisfying to see myself grow and being able to paint with a purpose.