The Lord is providing Himself with a king from among the sons of Jesse. But it is still a mystery to Samuel as to which son the Lord intends to choose. We would, perhaps, be right to think that the Lord has been preparing His servant, David, in the obscurity from which he is about to be plucked.
We can all be fooled by appearances. When seven of Jesse's sons passed before Samuel, he thought initially that the Eliab was God's choice for king. But the Lord tells Samuel that this is not the one. The Lord's purposes are not necessarily advanced by people that might merely look impressive in the eyes of the world. Even the prophet Samuel had to learn that God will surely advance His own candidate.
Very often, obscurity is a good training ground! David, the young man, was chosen when a shepherd with his father's flocks of sheep. The Lord Jesus Christ, lived in Nazareth, of which it had been said - 'can there any good thing come out of Nazareth?' (John 1:46)