If you have a car, then every once in a while, you probably have your vehicle checked to see if it's still up to safety and environmental standards. So you take your car to the garage and have it checked. Now, the garage will do some tests and eventually you'll get a nice paper showing what kind of maintenance they have done.
Nowadays, cars are complex, computerized machines. (The days of dad lying under the car to do some fixing with some elemental tools are all but gone.) This means that as a customer, you will have to rely on the professional capabilities and integrity of the garage. You'll have to trust that if the garage says the car is fixed and okay, it really is fixed and okay. Now imagine that you went to the garage, received the paper that your car is okay, go on the road, and your car breaks down. What would be your reaction? You'd probably hold the garage responsible for this, as they are the experts and you paid them to do a good job. What would your reaction be if they told you that they didn't have time to correctly solve your cars problems and did a 'quick fix', without them telling you?