Reason to act

"David was the son of Jesse, who was an Ephrathite from Bethlehem in Judah. Jesse had eight sons, and at the time Saul was king, he was already a very old man.

His three eldest sons had gone with Saul to war. The eldest was Eliab, the next was Abinadab, and the third was Shammah. David was the youngest son, and while the three eldest brothers stayed with Saul, David would go back to Bethlehem from time to time, to take care of his father's sheep.

Goliath challenged the Israelites every morning and evening for forty days. ...

... David left the food with the officer in charge of the supplies, ran to the battle line, went to his brothers, and asked how they were getting on. As he was talking to them, Goliath came forward and challenged the Israelites as he had done before. And David heard him. When the Israelites saw Goliath, they ran away in terror. “Look at him!” they said to each other.

“Listen to his challenge! King Saul has promised to give a big reward to the man who kills him; the king will also give him his daughter to marry and will not require his father's family to pay taxes.”"

~ 1 Samuel 17.12-25 GNB

So it seemed David's family could use some help to relieve their financial burden, because the father was very old already and all the older brothers were conscripted in the army. Plus, David had been assisting Saul and probably knew about Sauls' daughter. He would not mind marrying her.

Yes, David was already anointed by Samuel, but God also works through ordinary life circumstances. And this circumstance was what catapulted David into the next series of circumstances that brought him closer to be the anointed king.