Why hanger diagrams work
A hanger only balances when both sides weigh the same, and that is exactly what an equals sign means. When students take the same shape off both sides, they are doing the same thing to both sides of an equation, before any symbols get in the way.
That makes hanger diagrams a strong bridge from arithmetic to algebra in grades 6 to 8: one-step equations (6.EE.B.7), two-step equations like px + q = r (7.EE.B.4), and variables on both sides (8.EE.C.7). Once students can explain the picture, the algebra steps make sense.