What would a Jesse Tree devotion be without a story about Jesse?: 1 Sam. 16:1-13

And the Lord said to Samuel, “How long will you grieve over Saul, since I have rejected him as king over Israel? Fill your horn with oil and set out; I am sending you to Jesse the Bethlehemite, for I have decided on one of his sons to be king.” Samuel replied, “How can I go? If Saul hears of it, he will kill me.” The Lord answered, “Take a heifer with you, and say, ‘I have come to sacrifice to the Lord.’ Invite Jesse to the sacrificial feast, and then I will make known to you what you shall do; you shall anoint for Me the one I point out to you.” Samuel did what the Lord commanded. When he came to Bethlehem, the elders of the city went out in alarm to meet him and said, “Do you come on a peaceful errand?” “Yes,” he replied, “I have come to sacrifice to the Lord. Purify yourselves and join me in the sacrificial feast.” He also instructed Jesse and his sons to purify themselves and invited them to the sacrificial feast.

When they arrived and he saw Eliab, he thought: “Surely the Lord’s anointed stands before Him.” But the Lord said to Samuel, “Pay no attention to his appearance or his stature, for I have rejected him. For not as man sees [does the Lord see]; man sees only what is visible, but the Lord sees into the heart.” Then Jesse called Abinadab and had him pass before Samuel; but he said, “The Lord has not chosen this one either.” Next Jesse presented Shammah; and again he said, “The Lord has not chosen this one either.” Thus Jesse presented seven of his sons before Samuel, and Samuel said to Jesse, “The Lord has not chosen any of these.”

Then Samuel asked Jesse, “Are these all the boys you have?” He replied, “There is still the youngest; he is tending the flock.” And Samuel said to Jesse, “Send someone to bring him, for we will not sit down to eat until he gets here.” So they sent and brought him. He was ruddy-cheeked, bright-eyed, and handsome. And the Lord said, “Rise and anoint him, for this is the one.” Samuel took the horn of oil and anointed him in the presence of his brothers; and the spirit of the Lord gripped David from that day on. Samuel then set out for Ramah.

We anoint people with oil today, for various reasons. Here is the Oil of Catechumen, the Oil of the Sick, and Holy Chrism.