Brakes are an essential component on bicycles and cars for a few key reasons:

1. Safety – Brakes allow the rider or driver to slow down and stop the vehicle in a controlled manner, which is critical for avoiding collisions, especially in emergency situations. Without brakes, the vehicle would be uncontrollable.

2. Speed control – Brakes give the operator the ability to regulate their speed, both when needing to slow down and when going downhill. This allows them to maintain a safe speed for the conditions.

3. Load bearing – Brakes help to support the weight of the vehicle and rider/driver, especially when going downhill. The braking force helps to keep the vehicle stable and prevent it from gaining too much speed.

4. Legal requirements – In most places, it is legally required for bicycles and motor vehicles to be equipped with functional braking systems that meet safety standards. This is to ensure all vehicles on public roads can be operated safely.

So in summary, brakes are a critical safety and control mechanism that allow bicycles and cars to be used responsibly and prevent dangerous situations from occurring. They are an essential component for the safe operation of these vehicles.