Impaling has a damage multipler of x2, meaning that the damage that goes to the target after damage reduction is multiplied by two. In comparison, the cutting modifier is x1,5.
Furthermore, different types of damage interacts with the target's body differently. For instance impaling damage can reach the target's vitals while cutting is very effective against the throat.
Impaling though tends to be thrusts while cutting tends to be swings. Your swing and thrust damage is decide by your strenght, and swing damage tends to be higher than thrust, making it ideal to get through damage reduction.
Of course, a few weapons have the god of damage attacks, swing impaling. You do NOT want to get hit by swing impaling from a halberd of pickaxe.