Getting a complete and accurate symptom description, and reproducing the symptom, assures that you fix the symptom the customer wanted you to fix. A good damage control plan ensures that you won't make anything worse. Correct general maintenance shows the customer you care about quality, and often greatly reduces costs. A correct narrowing process will reduce costs, prevent further damage, and ensure that the root cause, rather than a peripheral symptom, is fixed. Proper repair or replacement of the defective component prevents further damage.
Testing is like inspection in the factory.
The few defects that escape the quality controls of the earlier steps are caught here, by showing that the symptom description you recorded and reproduced has been eliminated, and that no further problems have been created. Taking pride is a periodic maintenance item that ensures the quality of you as a troubleshooter and a human being. Preventing further occurrence is the utmost in customer service. The quality of the solution depends on the quality of the steps. One cool and easy bottleneck analysis test is to slow down a section of the system.