Why do people need to eat healthy foods? Can't people just eat what they want (big macs and candy bars) and take a multivitamin daily?

Unfortunately NO. Would you put the wrong fuel in your car? Food is the fuel that our bodies have to burn to create the energy for function. The nature of the questions on this forum clearly indicate that poor diet is making millions sick. Trouble is that they are being treated for headaches, abdominal pain, diarrhea, constipation, anxiety state, pains and aches, post nasal drip/allergies etc.