“break down their altars and dash their sacred pillars in pieces.” Sacred standing-stones were often set up as part of the worship of pagan gods, and pagan altars were a central part of pagan worship. But God has no tolerance for idols. They are harmful in many different ways. They are to be destroyed.
[For more on standing-stones, see commentary on Gen. 28:18. For more on idols being harmful, see commentary on Deuteronomy 7:5.]