Psychologically it's supposedly to terrorise Ukraine into surrender. Militarily it's to make them devote more resources to clearing the damage, to force Ukraine to move anti missile batteries away from the front lines and into cities and to make Ukraine use up its missiles on defence as well. What most commentators feel is the primary reason, though, is that the war is going so badly for Russia that nationalist groups in Russia are criticising the government and the military. Destroying civilian targets is a way for Russia to show those nationalists that it's doing something successfully instead of just getting its butt kicked on the front lines.