because they can tell you pretty much anything, not literally anything, but almost. If somethings wrong, then traces of it should show either in your blood or urine. Blood because it travels all throughout your body, and urine because it's one of the ways your body gets rid of the toxins or waste
Many things..Urine can show a urinary tract infection, too much sugar means diabetes, blood cells may mean a kidney stone or tumor in the tract. Blood tests can show liver, kidney and hormone abnormalities. Also show blood cell composition and anemia problems. Electrolyte balance and bleeding problems. The list is very long....Pregnancy, especially with abdominal pain issues.