I am sorry to hear you are undergoing some tests at the moment. You haven't said specifically which test you are having but I can only assume you are going to have a camera look at the inside of your bowels ( colonoscopy).
The bowel prep needed prior to the colonoscopy ensures the contents of the bowel is emptied properly. Foods such as white bread/ white rice that are low in fibre are more easily digested and pass through the system much more quickly, so this is why the diet changes in the days before the test are recommended.
