By setting up the carved image for themselves and having the resident Levite and his sons as priests at this shrine, the Danites ensure that Micah’s shrine lives on. But it is no longer a family shrine; now it is a shrine for the whole tribe! What makes all this so appalling, so grievous, is that the purpose of the shrine is not to worship God according to his covenant. The text explicitly says that they set up the carved image for themselves.
They erect this shrine to serve their greedy interests. They want to buy off the Lord, purchasing approval from him for the evils they have committed. But in reality, their actions show they have gone back to the foot of Mount Sinai and are once again worshipping the golden calf.1
30 And the people of Dan set up the carved image for themselves, and Jonathan the son of Gershom, son of Moses, and his sons were priests to the tribe of the Danites until the day of the captivity of the land.