Why did God command the prophet Ezekiel to bake with excrement?

The things Ezekiel was commanded to do in the sight of the people (in chapter 4) were symbolic actions describing the things that would come upon Jerusalem and the house of Israel. In 4:12, the prophet is commanded to enact (symbolically portray) the punishment described in 4:13: "Thus shall the people of Israel eat their bread unclean, among the nations where I [God] will drive them" (ESV). The prophet, repulsed by God's command to him to bake his bread "on human dung"—that is, bake the cake using human excrement as fuel for the fire—was permitted to substitute cattle manure instead (verse 15). The point here is that the prophet is, by symbolic actions, describing to the people the consequences of covenant breaking.