Coil springs allow the front and rear suspension system to move and adapt to different conditions of the road and they also support the weight of the vehicle and maintain ride height.
Over time, the coil springs can sag and they can also fracture and crack. Once the coil springs begin to fail, then they will need to be replaced.
Most manufacturers will cover the coil springs under the bumper-to-bumper warranty portion of the factory warranty. There are extended auto warranties available that will cover the cost pf replacing the coil springs under certain levels of coverage.