That is because only a specialized person can detect and solve a problem of a certain car in a fast and easy way. A self-repair can make even more problems to the car and more when it comes from people who tend to do not have such a large experience in this field.
Just imagine you car has some problems. There are many people who tend to provide a self-repair for the car while others follow the help of a recommended bmw garage. But in case you wonder which option is the best one for sure the professionals available can be the right answer.