Of course, you need them to stop your car! I found a great diagram that illustrates the basics of how your brakes work.

All the components are important, but in these times there are really just three that we need to be concerned about from a regular service perspective: the friction materials (brake pads and rotors), and the brake fluid itself.

When you push on the brake pedal it applies pressure through the master cylinder to the non-compressible brake fluid. The brake fluid transfers that pressure to the brake calipers- one at each wheel. That pressure is then applied to the brake pads- your friction material- which squeezes the brake discs (rotors) supplying the friction to slow you and your car down.

It is easy to understand that brake friction material is applied with pressure to steel discs creating friction that slows you down. Over time that friction material is worn away, and to a slower extent so is the steel in the discs. Every once in awhile the friction materials (brake pads) need to be replaced. At the same time, the steel discs may need to have their surfaces smoothed and trued on a brake lathe. Or the steel discs may be worn beyond their manufacturer’s minimum thickness. In this case, the discs need to be replaced to ensure safe braking.

The byproduct of the friction is heat, which is one reason there is a minimum thickness specified for your brake discs- they need to be able to dissipate the heat properly. One other part that is getting a lot of heat is the brake fluid- which is why brake fluid is rated by its boiling point. Brake fluid is hydroscopic, which means it absorbs water from the air. Over time this absorption lowers the boiling point of the fluid. In extreme cases, your brake fluid can boil in the calipers which generates steam, a gas, which is compressible. Bad! In that case, no matter how hard you push on the brake pedal there is no more braking force. 

Media credit: https://www.lesschwab.com/