What is the purpose of the light coating of flour in a three-step bound breading?
The flour as the first dredging step does help the rest of the breading stick. Think traction. It gives the egg something to hold on to, which then holds on to the breadcrumbs. You're right, the vast majority of recipes that call for this kind of breading call for a three step process. That's because it works better. I've done it with and without the initial flour dredge. With is better. Considerably.