It's necessary because drawing directly on the panel doesn't work well. So I draw on paper. During the transfer, the proportions of the drawing remain intact, but the grace and feeling are lost. Having invested all that time in the drawing, I hate to lose any of it. I have the option, if I've transferred my drawing with graphite, of working on the transferred drawing to improve it. On the other hand, a graphite transfer is usually completed with ink. Erasing graphite under dried ink leaves smudges on the panel, which I dislike on principle. Inking the drawing leaves heavy ink lines. For an opaque painting, that's no problem.
As I develop my skills, I'm going to want the option of making parts of my underpainting visible in the final painting. That's a good reason to explore other kinds of transfers that don't leave ink lines or smudged graphite behind.
One way I study drawing and painting outside the classroom is to read artists' blogs and watch their demonstration videos. A few months ago I watched a video demonstrating an oil transfer, made by Tacoma artist David Gray. I liked the idea of an oil transfer because the medium on the panel, from the transfer to the final layer, is oil paint -- nothing else. I decided to give it a try.