It appears that he wanted to facilitate easier conversation with the Lord. Israel, instead of having to go to the tabernacle, could just come to Ophrah and ask Gideon to inquire of God with his ephod.
But in this case, the word ephod
seems to function as a figure of speech, where the part stands for the whole. This ephod appears to be much more like an idol than a priestly garment. The amount of gold Gideon used suggests that in this case the garment clothed a molten, idolatrous image, an image that became the object of worship for the Israelites. This is indeed what we then read: Gideon put the ephod in his city, in Ophrah, and all Israel whored after it there. It became an idolatrous object of pagan worship. Most likely it served as a replacement of the shrine to Baal that Gideon had earlier torn down at the Lord’s command (Judges 6:25–32). “The irony and twistedness of his actions should not be missed. Instead of himself, an image of God, clothed with the Spirit of Yahweh (Judges 6:34), Gideon created his own image and clothed it with pagan materials."1
Thus, Gideon’s real intention seems to be to glorify himself. Though he had refused the title of ruler, in the ancient Near East, who founded cults, built temples, made golden images for worship? Canaanite kings. “The ephod does not memorialize Yahweh or even the people of Israel: it is Gideon's ephod and an ironic realization of Yahweh's warning against self-glorification (Judges 7:2)."2
27 And Gideon made an ephod of it and put it in his city, in Ophrah. And all Israel whored after it there, and it became a snare to Gideon and to his family.