It is not clear why David’s appearance is mentioned. Possibly it has to do with signifying his status in the family. The fact that he was ruddy suggests a reddish-brown colour, someone who spent a lot of time in the sun. Looking after sheep was not an enjoyable task. Presumably it was a job which most people avoided if they could. Along with confirming the work which David did, the mention of him as fine and handsome could serve as a contrast to the rejection of Eliab. Attractive qualities in an individual do not serve to qualify him for office, neither do they disqualify.1
12 And he sent and brought him in. Now he was ruddy and had beautiful eyes and was handsome. And the LORD said, “Arise, anoint him, for this is he.”