The job of a car suspension is to maximize the friction between the tyres and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. If a road were perfectly flat, with no irregularities, suspension wouldn’t be necessary. But roads are far from flat!
Even freshly paved highways have subtle imperfections that can interact with the wheels of a car; it’s these imperfections that apply force to the wheels. A bump in the road causes the wheel to move up and down perpendicular to the road surface.
Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose: contributing to the car’s road holding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations, etc.
These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the forces acting on the vehicle do so through the contact patches of the tires.
The suspension also protects the vehicle itself and any cargo or luggage from damage and wear; the design of front and rear suspension of a car may be different.