suppose you quarrel with your friend, if you say"你就是笨蛋一个",maybe,it's only a joking,your friend is not angry with you.but if you said"你就是一个笨蛋"，maybe,your friend feel insulted. so, I think "笨蛋一个" has a little fun,it's informal and not serious,but,the meaning of "一个笨蛋" is totally opposite. furthermore,let's look at these two examples "我愿化作青烟一缕" and "我愿化作一缕青烟",you will feel the former is more beautiful and more romantic than the latter,this is a rhetoric of Chinese style, it appears in many chinese poem. I hope I can help you!