Shocks wear progressively and the normal driver won't notice the difference.
Until you need to brake suddenly, swerve violently or even accelerate hard on an uneven surface.
The shock has just one job.
To keep the tyre's contact patch in full contact with the road at all times. If the inbuilt damping isn't working effectively, that won't happen.
I still have a couple of videos somewhere of testing done by Monroe Australia back in the early 80's.