A coil spring spacer sits directly between the spring and the chassis. It adds a small amount of ride height without replacing the spring itself. Off-roaders use them to clear larger tires. They are cheaper than full lift kits. But they do not improve suspension travel.
Rubber or polyurethane spacers can squeak over bumps as the suspension flexes. Metal spacers are noisy because they transmit road vibration directly into the frame. They are best for cosmetic lifting rather than performance tuning.