2 examples:
I'm pretty sure a placebo would have zero effect if antibiotics were needed to fight disease. Your immune response is struggling to overcome a species of bacteria - that's where the wonders of antibiotics come in.
You're overweight, stressed, a heavy smoker and you go to the doctor with chest pains, knowing that your blood pressure will be too high (sphygmomanometers are not that expensive). He gives you blood pressure pills, so you walk away less stressed with less 'in need of' a cigarette (so you smoke a little less). He could have given you placebos here, quite easily. By reducing your stress and smoking, your blood pressure falls to safer levels.