Shocks help dampen the spring oscillations from road shock which prevent your vehicle from swaying, squatting or bouncing when it, for example, hits a bump in the road. This would make your drive extremely dangerous as it would be difficult for you to handle your ride.
Both shocks and struts wear over time, and there are certain symptoms that indicate when it may be time for replacement:
If your vehicle is rolling or bouncing when you drive around corners
If your vehicle excessively dips when you are braking
If your vehicle sways when you hit a bump in the road
Shocks and struts have gas or liquid in them. As they age, they leak and their ability to help your vehicle keep contact with the road becomes compromised and they wear much more rapidly.