This effect can be quite detrimental to the stability of a vehicle. OK, here's an analogy. Hold up a compressed Slinky in the air with your hand. Now hold just one end and let the other drop. This what an automotive spring does if it doesn't have a shock absorber attached to it.
Struts are simply shock absorbers used as major structural members.
For struts, the shock absorber is placed inside the coil spring. In addition to saving space, it often costs less. Many cars use a strut design. Shocks and struts help control how fast the suspension is allowed to move, which is important for keeping the tires in contact with the road. Obviously, there is a lot more weight in the upper part of the car than unsprung weight in the lower part of the car.