What have food allergies got to do with not eating a healthy diet?

Restrics nutrients. Patients allergic to certain foods must insure that they are getting replacement calories, proteins, vitamins and minerals in their diet. For example, children with milk allergyneed to replace the protein and calcium. Confirmation of allergy by challenge is an option for many people, if the risk for anaphylaxis is low. Consultation with a nutritionist may be helpful.
Depends. If the questions is does a bad diet cause food allergies the answer is no. If the question is how can food allergies affect diet, then it would depend upon what food (s) is being avoided.