why put the chicken in skin side down first? only to brush with oil/herb mix???

You could do that, of course. I imagine the reason it's done the way described in the recipe is to make less likely to leave a dripping mess in the process of transferring the bird, as well as making sure any run-off gets caught in the pan. Turning and patting the skin dry before brushing on the rest of the mixture makes sure that the skin is better able to hold the oil mixture. If the skin is wet at all, the mixture is going to run off. If you prefer, of course you could brush the underside before putting it in the pan. It's just a small detail, and not actually something that is critical to the recipe success.