A bad strut makes noise and causes the vehicle to vibrate when it hits bumps. As the wear progresses, the noise often becomes worse. Replacing the strut helps the vehicle handle normally and quiets the noise.
Struts usually fail over time. Although the mileage between replacements varies, most need replacement at between 70,000 and 80,000 miles. Most drivers note a marked improvement in the handling of the vehicle when a bad strut is replaced.
Driving with bad struts increases the wear and tear on other components of the vehicle's suspension system. Ignoring the noise and vibration caused by a bad strut places additional stress on these components that can also cause them to fail.