Making a diagnosis and implementing a treatment plan is not something that happens instantly, but is a process. When we as doctors see a set of symptoms in a patient, we have a list of possible diseases or disorders in our mind. We then need to try and figure out which of those possibilities is the likely correct one. With many patients we can't determine which problem is the right one merely on symptoms alone. For example, let's say that a patient comes in for excessive drinking and urinating. Possible causes include diabetes, kidney disease, a urinary tract infection, or Cushings disease. Each of these are radically different disorders with equally different treatments. We can't simply try a treatment and hope it works. If we put a patient on antibiotics for an infection, we could be delaying needed treatment for diabetes.
So we run tests. Let's say that in this fictional patient the blood glucose and kidney values are normal, there are no abnormalities suggestive of Cushings disease, and there are bacteria and white blood cells in the urine. The conclusion here is that the patient has a urinary infection, and we place her on antibiotics. At this point some people would think that those other tests had been unnecessary, and might even ask for their money back. Yes, this happens, and it frustrates us vets. We have no way of knowing what the results would be before running them. Sorry, we usually forget to bring our crystal balls to work. The way I try to explain it is that these normal results are GOOD news. We have just ruled out the possibilty of several serious diseases, and this is something we should be happy about. The tests weren't pointless because we didn't know what the results would be ahead of time.
Coming to a diagnostic conclusion is rarely as simple as just examining a patient. Almost every symptom has at least a few possibilities, and we need to do tests to try and figure out which one it is. When we have those results we can determine which of our possible illnesses are more or less likely, and then proceed to the next round of tests or begin treatment. So when your own pet comes up with normal test results be happy about it, and realize that your vet is merely trying to help you by properly diagnosing the problem.