Evangelist Fred Brown used three images to describe the purpose of the law. First he likened it to a dentist's little mirror, which he sticks into the patient's mouth. With the mirror he can detect any cavities. But he doesn't drill with it or use it to pull teeth. It can show him the decayed area or other abnormality, but it can't provide the solution
Brown then drew another analogy. He said that the law is also like a flashlight. If suddenly at night the lights go out, you use it to guide you down the darkened basement stairs to the electrical box. When you point it toward the fuses, it helps you see the one that is burned out. But after you've removed the bad fuse, you don't try to insert the flashlight in its place. You put in a new fuse to restore the electricity.
In his third image, Brown likened the law to a plumbline. When a builder wants to check his work, he uses a weighted string to see if it's true to the vertical. But if he finds that he has made a mistake, he doesn't use the plumbline to correct it. He gets out his hammer and saw. The law points out the problem of sin; it doesn't provide a solution.