1 Samuel 17:54:
And David took the head of the Philistine and brought it to Jerusalem, but he put his armor in his tent.
Goliath’s armor would have been very valuable and useful. The Philistines had the upper hand over Israel, at that time in history, and they didn’t allow blacksmiths to operate in Israel. The Israelites were virtually devoid of weapons, due to the oppression of the Philistines. See 1 Samuel 13:19-22:
Now there was no blacksmith to be found throughout all the land of Israel, for the Philistines said, “Lest the Hebrews make themselves swords or spears.” (20) But every one of the Israelites went down to the Philistines to sharpen his plowshare, his mattock, his axe, or his sickle, (21) and the charge was two-thirds of a shekel for the plowshares and for the mattocks, and a third of a shekel for sharpening the axes and for setting the goads. (22) So on the day of the battle there was neither sword nor spear found in the hand of any of the people with Saul and Jonathan, but Saul and Jonathan his son had them.