Just to build on USAFguy22’s post: the purpose of most harmonic balancers (also known as harmonic dampers) is to dampen torsional vibrations in the crankshaft, not to make up for poor blueprinting and balancing.
When a piston pushes down on the crankshaft, it pushes down on one point, not evenly across the entire crankshaft. Pushing down on one point causes the crankshaft to twist a little and when you're done pushing it springs back, vibrating at its natural resonant frequency like a clock spring. The flywheel end of the crankshaft can’t resonate much because the flywheel dampens the vibrations on that end. To prevent resonances at the other end without resorting to another big heavy flywheel, harmonic dampers are installed in crank pulleys that are tuned to resonate out of phase with the crankshaft. By damping the crankshaft’s natural resonance at both ends, it will twist less, last longer, and can drive the camshafts more precisely, giving you more horsepower and a smoother running engine.
Those are very good things but they’re due to the crankshaft twisting less, not because the harmonic damper has made up for poor blueprinting and balancing.
STMPO pointed me to a webpage that explains the subject matter very clearly. From that webpage: Question: Will a harmonic damper solve an out of balance condition? Answer: No damper will compensate for an out of balance condition. The vibration damper is designed to reduce crankshaft torsional vibrations, not to balance the motor. It is a common misconception that a damper will "fix" an imbalance condition in the motor. This is absolutely not true.