Utility tower, normally does no damage on it's own
Will attract balls to it, holding them in place for splash damage towers like the bomb tower to do their work
The magnet tower normally does no damage on it's on. It's magnetic ability will attract balls to it, making it useful for clumping balls together for maximum effect from splash damage towers
The magnet tower Dark Power gives it a chance to reduce the health of all balls in range by 10%, which can be a massive amount of health. There's a limited number of magnet towers available though, so it's important to make their location count.