Also, you become confident to "do your own thing" and cook whatever variations you think up yourself, thus becoming a better cook rather than a mere recipe-follower.
And then on top of that Mastering is a great guide to cooking through a theme-and-variations approach. So once you have made a few of of Ms. Child's variations, you can recognize other recipes (from any source) that are similar, and can see how the techniques are the same. Also, you become confident to "do your own thing" and cook whatever variations you think up yourself, thus becoming a better cook rather than a mere recipe-follower. Which is the other main thing that Julia Child was trying to teach.